Union Pacific Yard Slugs and Other Hump Yard Motive Power

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This page was last updated on May 19, 2020.

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Union Pacific is one of a last Class One railroads to use electric trailers or boosters for yard switching duties. These units are generally called "slugs" by both railroads and railfans.

A slug is a locomotive frame with only traction motors and a ballast weight, sometimes with dynamic braking. The electric power to operate the slug's traction motors comes from an semi-permanently coupled power unit, which also serves as a controlling unit. Slugs have been converted by the railroads for a variety of types of service, including flat switching, hump yard switching, and to enhance the tractive effort in low speed road service. The first known electric trailers were built new for the Montana copper hauling Butte, Anaconda and Pacific in 1914 and 1915. They remained as the only slugs for 30 years, until New York Central converted both straight electric and early Diesel-electric units to slugs in the mid 1940s for service on Chicago area hump yards.

In the 1950s and 1960s yard slugs began to catch on with North American railroads, with examples being completed by Southern Railway System, Chicago and North Western, Canadian Pacific, Northern Pacific. The popularity of yard slugs grew in the early 1970s, when between 1970 and 1975, Louisville & Nashville, Norfolk & Western, and Santa Fe started their own fleets, along with Union Pacific. Also during this period, Southern, and the Chicago and North Western continued to add to the numbers of slugs they were operating.

During 1971 and 1972, Seaboard Coast Line received from General Electric 25 examples of large road slugs for use with new U36Bs. The peak of all slug production, both for yard and for road service, came in 1979, when 62 slugs were completed, mostly for Conrail. By some accounts, as many as 500 slugs have been completed. Slugs have been rebuilt from a great variety of original locomotives, from small GE 45-ton center cabs for industrial users, to all manner of switchers of almost all models, and from all builders, to former road locomotives such as RSD15s, SD24s, and SD40s for Santa Fe, C628s and C630s for Southern Pacific, and Fairbanks-Morse H24-66 Train Masters for Norfolk and Western.

Union Pacific chose to enter the field of yard slug operation in small numbers, with only six rebuilt from GP9Bs and two rebuilt from SD24Bs. UP's merger partner Missouri Pacific rebuilt its 23 yard slugs from retired EMD yard switchers.

The earliest Diesel switchers on Union Pacific were the EMC/EMD NW2s; 36 had been delivered by July 1942. The ALCo S-2s and Baldwin VO1000s came in 1943 and 1944, and more NW2s, the Fairbanks-Morse switchers, and newer Baldwin DS-4-4-10s came in the late 1940s. The ALCo S4s and EMD SW7s and SW9s came in the early and mid 1950s. By the late 1940s, Union Pacific found itself in need of a special type of yard switcher; a switcher that could be used to move entire trains over its new hump yards.

Pocatello, Idaho, was Union Pacific's first hump yard and was opened in 1947 (it was called a retarder yard when first completed). The hump yard at North Platte, Nebraska, was opened in 1948 as UP's second hump yard, but it was the railroad's first automatic classification yard. Motive power for the new hump yards at first consisted of single new EMD NW2s. As rail traffic and train length grew, UP began using double-unit sets of NW2s as hump power, with their attendant full, six-man switch crews. In a move to reduce the labor costs of double crews, UP went looking for more powerful hump yard switchers, and found them at Baldwin in the form of AS-616 heavy duty road switchers. By late 1951, UP had purchased six-axle, 1,600 horsepower Baldwin AS-616s specifically for assignment to their two hump yards at North Platte, Nebraska, and Pocatello, Idaho, along with other heavy switching duties, including flat switching in Ogden, Utah. There is photographic evidence that UP used its Fairbanks-Morse H15-44s and H20-44s for a short time on the hump at Pocatello.

During the late 1960s, assigned power for the Pocatello hump yard was GE U25Bs, operating back to back. Reserve power for the Pocatello hump consisted of two AS-616s and four TR5 cow-calf sets. North Platte used two TR5 sets along with regular road power. By 1970, four SD7s (UP 454, 456, 458, and 459) were assigned to North Platte, along with most of the 2900-class ALCO C630s. The U25Bs operated at Pocatello until they were retired in September 1972, and the C630s remained at both Pocatello and North Platte until their retirement in 1973. After the C630s were retired, the ten 2800-class U28Cs were assigned in their place, where they also remained until their retirement in 1979-1981.

An additional hump at North Platte was opened in 1968. First called Edd H. Bailey Yard (later Bailey East), this completely new yard, dedicated to eastbound traffic, was state-of-the-art and the largest hump yard in the country. With its completion, the original 1948-built hump yard became the westbound hump yard. In 1974 the hump yard at East Los Angeles was completed, and in 1978 the hump yard at Hinkle, Oregon, was finished.

With the completion of the new westbound hump at North Platte in early 1980 (replacing the original yard built in 1948), UP began using SD40-2s in both hump and flat switching duties. The SD40s and SD40-2s were used to replace, at some of the larger yards, varying combinations of both single and double GP9s, GP20s, GP30s and GP35s. Several of the lower numbered 3000-class SD40s were assigned to yard service, and the eight SD40Xs (3040-3047) were assigned to North Platte and Kansas City for heavy switching from 1975 to their retirement in the mid and late 1980s.

UP 3702, 3703, 3708, 3709 were assigned to hump yard service in May 1987. The change in service included removal of their pilot snowplows. UP 3702, 3703 were assigned at Hinkle and UP 3708, 3709 were assigned at North Platte. After the merger with MoPac in 1982, UP began using, until their retirement in 1987 and 1988, the ex-MoPac 2009-2073 series GP38-2s in both local and yard service. UP also began using some of the non-dynamic braking equipped SD40-2s (4202-4215) at Bailey Yard in North Platte, Neff Yard in Kansas City, and Centennial Yard in Fort Worth. UP began replacing SD40s and SD40-2s in yard service with GP38-2s in mid-1990, using the leased 1800-class and the 2100-class ex-MP units, and the ex-MKT 2300-class units.

UP's First Yard Slugs

The first slugs completed by Union Pacific were for use at the yard complex at North Platte. At the time UP had only three hump yards: North Platte, Pocatello, Idaho, and Los Angeles, California. The two hump yards at North Platte make up what is Union Pacific's largest classification yard, and which according to the Guinness Book of World Records, is the largest in the world. During the 1970s there were actually two hump yards at North Platte; one was the old yard, called the westbound yard, completed in 1948 as UP's first automatic classification yard, the new Edd H. Bailey eastbound yard completed in 1968. A new westbound hump and classification yard was completed in 1980.

The UP's first yard slugs, with road numbers S1-S3, were built in 1973 and 1974 from three retired GP9Bs, and initially mated for hump service with three SD7s at Bailey Yard in North Platte, to fill the gap left by the retirement of the C630s. Three more retired GP9Bs were rebuilt into slugs: S4 and S5, were built in 1975 and 1976 for use at the hump at East Los Angeles; and the S6 was built in 1976 for the hump yard at Pocatello. The initial assignment of the 446/S5 ten-motor slug set to Los Angeles upon its completion in March 1976 allowed the reassignment of road units back to their intended service, such as GP30 720 which was on the East Los Angeles hump, with SD24 415 in September 1975. By the time of the completion of slugs S4-S6, UP had found that additional horsepower was needed for the hump sets, and these later slugs were mated with SD24s instead of SD7s. Whether mated with an SD7, or with SD24s, UP called these combinations ten-motor slug sets. Two SD24Bs were converted to slugs S7 and S8 in 1978 and 1979 for use at UP's new-in-1978 hump yard at Hinkle, Oregon, which opened in February 1978. They too were mated at first with SD24s, forming UP's first twelve-motor slug sets.

Union Pacific chose a low-cost, minimum effort to build its first yard slugs, six four-motor units with road numbers S1-S6. The railroad selected retired GP9 B-units and removed their engines, main generators, and other mechanical components, but retained their full carbodies. The electrical controls, cabling, and wiring, were given minimal attention (which would later shorten the slugs life span). The dynamic brake controls and grids were removed, the braking grid vent openings were sealed, and the 48-inch dynamic braking fan was removed, as were the radiator intake screens and the radiator cooling fans. From a distance, theses new slugs were little changed in appearance from their GP9B predecessors. Internally the ex-GP9B four-motor slugs retained their electrical cabinets and traction motor blowers. To make up for the lost weight of engine, generator, and other mechanical components, UP's Omaha shops poured 17 cubic yards of concrete into 5-foot diameter circular containers, and installed the containers directly to the frames of the new slugs as ballast.

Union Pacific's first yard slug, road number S1, was completed in October 1973. After completion, the new slug sat at the Omaha shop waiting for a matching SD7 slug power unit. SD7 459 was the first slug power unit completed, and was mated with the new slug in December 1973. The pair then was assigned to North Platte for tests. With the test complete, a second slug and SD7 set was completed. Slug S2 was finished in June 1974 and mated with SD7 454. This new ten-motor slug set was first assigned to North Platte. The program continued as slug S3 was released from Omaha shops on October 9, 1974, mated with SD7 458 and also sent to North Platte for service.

The success of the yard slug program soon led to additional retired GP9Bs being rebuilt to slugs in mid 1975. In addition to the North Platte yard complex in central Nebraska, UP also operated hump yards at East Los Angeles and Pocatello, and these two other yards were in need of the same economies provided by operating the slug sets at North Platte. The 1,500 horsepower SD7s had proven to be under-powered, although still usable, for the long strings of cars being moved over the hump at North Platte. The railroad found that a ten-motor slug could not directly replace two road units coupled back to back, and that the higher horsepower of two powered units was needed after all, contrary to the initial thinking that what was needed was the tractive effort from the extra axles. Because the power at the East Los Angeles hump was usually two road units coupled back to back, the railroad knew that another SD7 would not work in that assignment. SD24s 402 and 446 were regularly assigned to the LA hump, coupled with GP9Bs, so in mid 1975, the 402 was sent to Omaha to be converted to a higher horsepower slug power unit. UP Slug S4 was completed in September 1975. The mated UP 402/S4 set was released from Omaha Shops on September 28, 1975, and sent back to Los Angeles on October 4th.

Within three months of the new 402/S4 combination being assigned to Los Angeles in late 1975, SD24 446 was sent from LA to Omaha to be mated with the next yard slug. UP slug S5 was completed and mated with 446 in March 1976, and the newest ten-motor slug set was assigned to Los Angeles, joining the 402/S4 combination. S5 was a bit heavier than the previous four slugs, weighing in at 260,000 pounds compared to the earlier units with 235,300 pounds.

The sixth and final four-motor slug built from a retired GP9B was the S6, completed in late August 1976, and mated with SD24 429. The 429/S6 slug set was assigned to Pocatello in mid September 1976 in apparently the first use of slug power at that location.

Union Pacific soon found that even the combination of a four-motor slug and a 2,400 horsepower SD24 was insufficient to push long trains up the humps at its three hump yards, and that more tractive effort was needed to better match the higher horsepower. In mid 1977, nine months after the completion of four-motor S6, the railroad began design work on a six-motor version to increase the available tractive effort. Six-motor slugs S7 and S8 were built from retired SD24Bs by removing their engine and radiator hatches, and had their car-bodies cut down to 60 inches above the walkway to improve visibility from the controlling power unit. Ballast in the form of 28.5 cubic yards of concrete was placed in both slugs to replace the weight of the removed mechanical components.

Work on slug S7 began on in early November 1977, and the new six-motor slug was completed in late January 1978. By late April 1978 slug S7 was mated with SD24 405 and released for testing of this new twelve-motor slug set concept. There was some discussion of assigning the new twelve-motor slug sets to the new hump yard just completed at Hinkle, Oregon, but the newest set, the combined 405/S7, was assigned to North Platte instead. Six-motor slug S8 was completed in December 1978 and mated with SD24 419. This newest twelve-motor slug set was first assigned to the new yard at Hinkle, but within two years the set was re-assigned to North Platte, joining the 405/S7 set, and SD24 and SD24B 410/408B.

The slow speed and heavy pushing service in which the slug sets were working necessitated the need for a lot of traction sand. In an effort to increase the sand capacity of the slug sets, in October 1980, slug S7 had DD35 style sandboxes applied at Omaha. These were the same design exterior, five-inch thick, sand boxes which were applied to the DD35s and DD35As in the late 1970s, locating the sand boxes, and their fine dust particles, away from the electrical control compartments at the end of the big double diesels (the sand boxes applied to the slugs may have been removed from retired DD35s, which were being retired from the railroad at this time). Upon application of the larger sand boxes, UP 405/S7 was returned to North Platte, and resumed work along side 419/S8 and 410/408B. The large exterior sandboxes were also applied to four-motor slug S4, also at Omaha in early October 1980, for its use in track sled service.

Yard Slug Power Units

Union Pacific's SD7s were selected as the first slug power units in early 1973 because they were already assigned to hump service at North Platte. The SD7s were purchased in mid 1953 for the Iron Mountain iron ore trains in Utah. In 1968 the SD7s had been replaced on the Iron Mountain trains by new SD45s, with half of the SD7 fleet going to North Platte and the other half being to local service between Salt Lake City and Ogden and the Clearfield Freeport Center, which during the 1970s was the single largest traffic generating point on UP (UP 457, also as previously numbered 782, remained at Laramie for local service on the Coalmont Branch until its retirement in 1978.) With the near completion of the first four-motor slug, number S1, in late September 1973, UP looked into the assignment of at least one ten-motor slug set (a six-motor SD7 and a four-motor slug) to the switching of the Iron Mountain iron ore trains near Cedar City, Utah. As the first three UP yard slugs were completed, between December 1973 and October 1974, they were mated with SD7s 459, 454, and 458, and all three sets were instead assigned to North Platte.

By late 1980 the SD7 slug power units had reached the end of their useful lives and were in need of retirement. To replace the SD7s, UP selected SD40s with their 3,000 horsepower. By this time, the SD40 fleet was at least 15 years old, and had been replaced in priority freight service by the newer SD40-2 and C30-7s. UP 3034 was selected as a slug power candidate for the test, and after modification, the SD40 was mated with slug S8 in early January 1981. In late March 1981 the test was completed and within two weeks, in early April 1981, another SD40, UP 3035, was mated with the other six-motor slug, S7. Both new twelve-motor slug sets were assigned to North Platte.

SD40 3031 was mated with slug S3 from April 1981 until the slug was wrecked and retired in September 1982; the 3031 then replaced SD7 454 in its combination with slug S2. S2 was retired in mid 1985, and 3031 then replaced SD24 446 operating in track sled service with slug S4. During August 1982, SD40 3033 was modified as a slug power unit by North Platte shops and mated with four-motor slug S1, replacing SD7 459, which had been retired during July 1982. This new ten-motor slug set was to be assigned to Salt Lake City, but in October 1982 the 3033/S1 set was sent to Los Angeles instead, to replace slug set 402/S5, already assigned there. SD24 402 and slug S5 were separated in early November 1982 at Salt Lake City, with 402 being retired and replaced by SD40 3032. The 3032/S5 set were returned to the slug's original assignment at Los Angeles. Because the SD40s were able to operate with either the four-motor slugs or the six-motor slugs, UP regularly re-assigned the SD40s to different slugs, as maintenance schedules necessitated, so it was possible to see each of the six SD40s assigned to slug service, road numbers 3030-3035, with almost any of the various slugs operating on the railroad, at any of the four hump yards, North Platte, Pocatello, Hinkle, or East Los Angeles.

During the mid 1980s, two of the three remaining four-motor, ex-GP9B slugs, S4 and S6, were subjects of a study to replace their GP9 electrical switching gear with the switching rear from retired SD40s, to better match their SD40 power units. The study proved that the modification would be too costly. With the retirement of SD24 429 at Salt Lake City in March 1985, slug S6 was without a power unit. SD40 3030 was selected for this duty. However, before the two were mated together, the carbody of slug S6 was cut down to 77-inches above the walkway to improve the visibility from the power unit. The work was completed in May at Salt Lake City and the set was assigned to Pocatello for service, until the retirement of S6 in late August 1989. As the last of UP's original slugs were removed from service in July and August 1989, the SD40s were all returned to normal heavy switching service, and the six units were themselves all retired between January 1987 and October 1991.

Track Sled Service

In an innovation to reduce track maintenance costs, in 1979 and 1980 Union Pacific developed a method to refurbish the track substructure by removing the fouled ballast from under the track, with the ties and rail remaining in position. The piece of machinery used to accomplish this was called a "track sled". The track sled was placed under the ties by jacking up the combined rail and tie structure and attaching the sled to a locomotive, with the locomotive riding atop the rails and ahead of the sled. In this service, the ten-motor slug set was used to pull a sled along under the ties of a stretch of mainline track, plowing the fouled ballast out from under the track. After the sled had been used to clear a particular section of track, the ballast was taken up by other machines from along side the ends of the ties, cleaned, and redistributed and tamped back to its original location. The operation required a locomotive with very high tractive effort because of the slow speeds. High pulling power was needed to pull the sled along, lift the track, and remove the ballast, all at the same time. With the completion of the twelve-motor slug sets, UP had an excess of the smaller ten-motor slug sets. A ten-motor slug set (402/S4) was tried in this track sled service in Wyoming during the summer of 1980 and found to be successful.

One of the limitations of using the ten-motor slug set in track sled service was that the former GP9B slug regularly ran out of traction sand. In October 1980, the slug S4 was equipped with exterior sand boxes of the same design as used on the DD35s in the late 1970s. The 402/S4 set was used in track sled service during the summers and either stored or used as stand-by at various hump yards during the winters. In October 1982, SD24 402 was retired and slug S4 was then mated with 446, after the 446 had been separated from slug S5 (S5 had been mated with SD40 3032). The set was used briefly in track sled service during 1983, but by midyear, UP 446/S4 were stored at North Platte, with plans for the slug set to be moved to Cheyenne for a test in flat switching in that important yard. By December 1984 the set had been assigned to Los Angeles. In late February 1985 the 446/S4 set was moved to Pocatello in preparation for another season of ballast sled service, leaving 3033/S1 as the only slug set in LA. In May 1985 the 446 was retired and slug S4 was mated with SD40 3031 and used at North Platte in regular hump yard service. The track sled program continued through the summer of 1985, with the SD40 and slug combination of 3032/S5 being used.

UP 402, and its paired yard slug UP S4 were used as lead unit in UP's track sled program from summer 1980 to October 1982. Then UP 402 was retired and replaced by UP 446 during the track maintenance seasons of 1983 and 1984. UP 446 and S4 were replaced by SD30 3032 and S5 in track sled service for the 1985 season.

Remote control (RCS) capability was used to allow the maintenance of way personnel to control the ten-motor slug set as it pulled the track sled along clearing the fouled ballast from under the track structure, during what is known as track rehabilitation and ballast cleaning.

Union Pacific was late in adopting the concept of ballast cleaning and then tried to do it themselves with their own equipment. The Union Pacific's way of doing it on their own went away with the mergers and today ballast cleaning is done by contractors. The concept of a track sled to remove the ballast from under track has been replaced by a mechanism that many have compared to a big chain saw that does a better job by scooping the fouled ballast from under the track. All track maintenance contracting companies offer similar services. Research suggests that Loram began the concept in the late 1950s using the patented Mannix sled and plow. Here is a good description of the concept from the Loram Wikipedia page:

"The company's first mechanical products were the Mannix Sled and Mannix Plow, both developed in the late 1950s. The Mannix Sled was a device towed behind a locomotive which raised the rails and ties and cleared the ballast between the ties (a process known as "skeletonizing"). The Mannix Sled would be followed by a work crew that manually refilled the empty space with clean ballast. The Mannix Plow was a device that lifted both rails and ties, while three blades passed below them and removed all the ballast. This left the ties and rail lying on bare earth; a large work crew followed, lifting the rails again and replacing the ballast. The concept of lifting the rails and ties was counterintuitive, but it revolutionized rail bed rehabilitation."

(There are several Youtube videos online showing the ballast undercutting process.)

Slug Retirements

The ex-GP9B slug retirements began in 1982 when S3 was involved in a wreck at North Platte. Because of the abundance of available secondary road power that could be used for switching duties (the GP30s and GP35s) the slug was retired rather than repaired, and was scrapped by UP at Omaha in early September 1982.

Slug S2 was separated from SD40 3031 in July 1985 and removed from service. It joined SD24 446 (the last SD24 on UP, retired in May 1985) in dead line at Salt Lake City. S2 was retired on February 9, 1987 and sold for scrap to Southwest Railroad Car Parts in Longview, Texas in April 1987.

Six-motor slug S8 was wrecked in North Platte yard in late April 1985, while mated with SD40 3034. The two units were separated and the S8 was removed from service at North Platte. It was later moved for storage at Council Bluffs in late May 1986. Four-motor slug S1 was removed from service in June 1986. Both S1 and S8 were retired on February 9, 1987, and both units were sold for scrap to Aaron Ferer and Sons, a scrap dealer adjacent to UP's shops in Omaha, in November 1987.

UP slug S5 was removed from service in August 1988, and stored at North Little Rock, Arkansas. It was retired on April 24, 1989. The last former GP9B slugs, S4 and S6, were retired on August 29, 1989.

The era of yard slugs built by UP came to an end on January 17, 1990 when four-motor slugs S4, S5, S6, and six-motor slug S7 were sold for scrap to Pielet Brothers Metal and Salvage in McCook, Illinois, which is better known as the scrapper for old units traded to EMD. The four units were delivered during February and March 1990 and cut-up.

MoPac Switchers and Slugs

Union Pacific's 1982 merger partner Missouri Pacific also caught the yard slug fever and built 23 yard slugs, road numbers 1400-1422. The MoPac slugs were built from retired SW7s, SW8s, and SW9s in a program that started in late 1978 and ended in late 1982. The conversion of these units from switchers, including completely new, cut-down carbodies, was done by MoPac's Pike Avenue shops in North Little Rock, Arkansas. Upon completion, the MoPac yard slugs entered service spliced between MP15s or SW1500s at various flat and hump yards across the MoPac system.

Missouri Pacific's own yard slug program began with the completion of MoPac yard slug 1206, rebuilt from SW7 1206, and mated with 1,500 horsepower SW1200 1275. MoPac yard slug 1206 was later renumbered to MP 1400. As they were completed, each yard slug was mated with a Missouri Pacific MP15 (later called an MP15DC to distinguish them from the later AC version called the MP15AC). MoPac 1545 was the first MP15DC to be mated with MoPac's new yard slugs, completed in December 1978.

As MoPac's slug program ended, in January 1983, MoPac slug 1423 was in the process of being rebuilt from SW9 1234. The partially converted yard slug was sold for scrap to Gray Supply, a scrap dealer adjacent to the North Little Rock shop on April 12, 1985.

By the end of the first quarter of 1984, the combined Union Pacific and Missouri Pacific was beginning to benefit from their late 1982 merger. The railroad was doing less carload switching in each of their yards because many of their trains were being more efficiently pre-blocked, or were run-through operations. During early 1984 MoPac's large yard at Dupo, Illinois, had been almost entirely shut down and the second hump at Kansas City's Neff Yard was taken out of service. Along with the reduction in use in the switchers went UP's diminishing need for the ex-MoPac yard slugs, with many being stored. In late January and February 1986, all of MoPac's remaining 152 SW9s and SW1200s were retired. These switchers were replaced in switching service by more versatile GP15-1s, B23-7s, GP35s and GP38-2s.

The actual assignments of MoPac's 23 slugs, as of October 1984, were as follows. Six (MP 1401, 1404, 1406, 1407, 1408, and 1409) were assigned to Neff Yard in Kansas City; four (MP 1411, 1413, 1414, and 1421) were assigned to St. Louis. Five slugs (MP 1400, 1403, 1412, 1415, and 1416) were at North Little Rock; three slugs (MP 1417-1419) were being used at Houston; and the final five (MP 1402, 1405, 1410, 1420, and 1422) were at Centennial Yard at Fort Worth. The slugs were working in sets with either MoPac's MP15DCs or GP15-1s.

In December 1984 Missouri Pacific assigned ten of its leased UP SD45s to hump service, replacing sets made up of MP15/slug/MP15 combinations and single MP15/slug sets (these ten SD45s were part of a twenty unit group leased to MoPac by UP in March 1984). Two SD45s (UP 21 and 36) were assigned to Kansas City; another set of two (UP 6 and 41) was assigned to Fort Worth; and the remaining six units were assigned in three sets of two to the humps at North Little Rock. The UP SD45s replaced slugs sets made up of a GP15-1 and a 1400 class MoPac yard slug, allowing them to be reassigned to other switching duties. Soon after, other units replaced the SD45s at North Little Rock and some of the big six-axle units were reassigned to the hump at Centennial Yard in Fort Worth. Others were removed from service and stored, with at least one unit, UP 21, out of service and stored at Council Bluffs in July 1985. The ownership of all 20 SD45s was transferred from UP to MoPac in March 1986.

The UP SD45s proved to be low reliability units, so by June 1986 only six UP SD45s (UP 4, 6, 17, 32, 41, and 48) remained in hump service, all at Fort Worth, although only three of them (UP 6, 32, and 48) were actually in service. At that time, two former MoPac SD40-2s (UP 4214 and 4215) were assigned to the Fort Worth hump yard to replace the last three SD45s. The three SD45s joined the other three in stored unserviceable status until all six (the last SD45s on UP) were retired in January and February 1987. EMD's SD45s, with its trouble-prone 20-cylinder have proven to be a high maintenance locomotive, and the UP SD45s assigned to the Fort Worth hump yard were found to fit the pattern, possibly because a twin-unit set was grossly over-powered for the assignment and were only being taxed to a fraction of their capacity. When the last three SD45s were removed from hump service, they were replaced by sets of freshly repainted former-MP SD40/SD40-2s, along with dual slug sets of MP15s and slugs, coupled as MP15/slug/MP15/slug.

All of the 23 ex-MoPac yard slugs were repainted to UP's yellow and gray paint, with S-series road numbers. MP 1407, 1408, and 1409 were first painted while assigned to Houston to the earlier MoPac version of yellow and gray paint, with their MoPac road numbers, in the MoPac style lettering, referred to by some as the "North Little Rock style" lettering. These three slugs were painted in May and July 1985. In December 1985 through September 1986 seven other slugs were painted in UP's gray and yellow, with the UP S-series numbers in the North Little Rock lettering, all others were painted in standard UP paint and standard UP lettering.

In January 1986 five ex-MP yard slugs were leased to Alton & Southern Railroad, in the first use of slugs on that terminal switching road located in the St. Louis area. Missouri Pacific had purchased a half interest in the Alton & Southern, along with Chicago & North Western, from Aluminum Corporation of America (Alcoa) on May 9, 1968. C&NW later sold its interest to the SP-controlled St. Louis Southwestern (Cotton Belt). The new MoPac/C&NW owned A&S repainted its ALCo RS-2/3s (which had dieselized the railroad) to a new paint scheme that combined MoPac blue on the cab and frame with C&NW yellow on the hoods. The numbers were in MoPac's style and the A&S herald was a hybrid of the C&NW bar across the MoPac buzzsaw, with Alton & Southern spelled out in the bar. Within a year, A&S began trading in its RS-2s and RS-3s for new EMD SW1500s. A&S 1500-1505 were delivered in May 1969; A&S 1506-1511 in February 1970; and A&S 1512-1517 in April 1971. In December 1972, MoPac itself purchased their 1518-1521 (numbered after the A&S road numbers) as the only SW1500s that MoPac was to own. The Alton purchased an MP15DC, number 1522, in October 1980, which was numbered after the four MoPac SW1500s. This new A&S unit was also painted in the A&S scheme of yellow hood and blue cab.

Included in the January 1986 assignment of MoPac slugs to the A&S were S21-S25. S21 was mated with A&S SW1500 1501, and S24 was mated with A&S SW1500 1504. This assignment of was the first for these slugs as UP-numbered units, as all five were repainted from their MoPac paint and number in preparation for the assignment. All five units were actually on the A&S by late March 1986. In the late 1990 systemwide re-assignment of switching motive power, three of the ex-MP slugs were retained in their lease to the Alton.

In a test of better utilization of its yard slugs and yard switching motive power, in early 1986 Union Pacific mated ex-MP yard slug S24 with an SD40 and operated the set in St. Louis transfer service. The use of the SD40 allowed two MP15s to be reassigned to other switching duties. This test only showed the limited utility and forced slower speeds required in the operation of the yard slugs, especially since the ex-MP yard slugs equipped with switcher trucks, and soon the pair was separated. In August 1986, 15 of the 23 former MoPac slugs were stored out of service. In November 1986, of the remaining eight slugs, S9 and S10 were far away from their original MoPac home trackage, assigned with ex-MoPac MP15DCs 1318 and 1319 to flat switching duties at the hump yard in East Los Angeles, California (a twelve-motor UP slug set made up of SD40 3032 and S7 were being used on the East Los Angeles hump).

Early 1987 saw additional reassignments of the former MoPac yard slugs. S19, freshly repainted from MoPac 1410, was assigned along with likewise freshly repainted MP15 1391 and 1392 (making a 3,000 horsepower, 12-motor slug set) to the hump yard in Pocatello, Idaho. In UP's ever-changing attempts at better utilization of its motive power fleet, UP S13, freshly repainted from MoPac 1404, was briefly sent to the large, former Western Pacific yard at Stockton, California, where it was mated with MP15 1356 for an unsuccessful test in flat switching duties. The 1356/S13 set was then very briefly assigned to flat switching at Cheyenne, Wyoming. In both these tests, the low horsepower of a single MP15 just didn't work out. In the case of Cheyenne, the set was replaced by an unusual combination of a B23-7 and SW10.

By late 1988, the use of the former MoPac yard slugs had fallen to the level that saw 11 of the 23 units to be stored at the giant Downing B. Jenks shop complex in North Little Rock, Arkansas. Included in the storage lines were S11, S13, S14, S15, S16, S17, S18, S27, S28, S29, and S30. By December 1989 UP Slug S12 had been moved to Albina Yard in Portland, Oregon.

As mentioned previously, Union Pacific regularly moves and re-assigns its fleet of switching locomotives and yard slugs, trying to get optimum usage. In December 1990, in a move to put SD40-2s assigned to yard switching service back on the mainline, UP assigned GP38-2s and GP15-1s to the various hump yards over the system. GP38-2s in the 1800 class, 2100 class, and the former MKT units in the 2300 class were assigned to the yards in Hinkle, Pocatello, Fort Worth, and both the east bound and west bound hump yards at North Platte. The new assignments called for the return to service of all 23 yards slugs. S9 and S10 were to retain their Los Angeles assignment. S11-S13 were assigned to go to Neff Yard in Kansas City. Eight slugs, S16-S19, S22, S23, S25, and S26, were sent to North Little Rock for service. Three (S20, S21, and S24) were already in lease service on the Alton & Southern. S27 and S28 were mated with GP38-2s 1800 and 1801 for trim switching duties at the hump yard at Hinkle. Finally, three slugs (S29, S30, and S31) were assigned to the yard complex at North Platte, intended to operate with ex-MKT 300-class GP38-2s 2346, 2347, and 2348. By late December 1991, the rigorous service at North Platte had proven to be too much for these little workhorses, and all three, along with S28 from Hinkle, were again in storage at Armstrong Yard in Kansas City. Because they are leased locomotives, the 1800s assigned to Hinkle and Pocatello are not wired to operate with the slugs, so when an 1800-class GP38-2 and a slug are seen together, the slug is being operated as only a braking trailer.

Union Pacific has found that the utility of ex-MoPac yard slugs is even more limited than its earlier (now retired) yards slugs rebuilt from GP9Bs and SD24Bs. The major limitation being the switcher-type trucks. The use of the yard slugs was, and still is, severely limited by their slow acceleration with long and heavy cuts of cars. This speed limitation is okay when pushing up to a hump, but is not acceptable for flat switching, including the trim work in the bowl tracks at the system hump yards. In the case of the earlier UP-built slugs, the retirement of the former Missouri Pacific yard slugs was forced by the condition of the electrical systems, which were not redone when the slugs were built. Today, Union Pacific prefers to use two powered units on the hump yards, although the slugs also still see some limited use at hump yards system wide.

UP Ten-Motor Slug Sets

Power Unit
S1 SD7 459 Dec 1973 - Jul 1982 UP 459/S1 were first assigned to North Platte, then to East Los Angeles.
  SD40 3033 Aug 1982 - Jun 1986 UP 3031/S1 were assigned to East Los Angeles until S1 was retired.
S2 SD7 454 Jun 1974 - Nov 1982 UP 454/S2 were first assigned to North Platte.
  SD40 3031 Nov 1982 - May 1985 UP 3031/S2 were assigned to Pocatello until S2 was retired.
S3 SD7 458 Oct 1974 - Feb 1981 UP 458/S3 were first assigned to North Platte.
  SD40 3031 Apr 1981 - Sep 1982 UP 3031/S3 were assigned to Pocatello until UP S3 was wrecked and retired.
S4 SD24 402 Sep 1975 - Oct 1982 UP 402/S4 were first assigned to East Los Angeles, until reassigned to ballast sled operations in Nebraska in November 1980.
  SD24 446 Oct 1982 - May 1985 UP 446/S4 assigned to ballast sled operations system-wide.
  SD40 3031 Jul 1985 - late 1985 UP 3031/S4 were assigned to North Platte.
  SD40 3031 late 1985 - Aug 1989 UP 3031/S4 were assigned to Pocatello, until S4 was retired.
S5 SD24 446 Mar 1976 - Oct 1982 UP 446/S5 were first assigned to East Los Angeles.
  SD40 3032 Nov 1982 - Jun 1983 UP 3032/S5 were assigned to East Los Angeles.
  SD40 3034 Jun 1983 - May 1984 UP 3034/S5 were assigned to East Los Angeles.
  SD40 3033 May 1984 - Apr 1986 UP 3033/S5 were assigned to East Los Angeles until S5 was retired.
S6 SD24 429 Sep 1976 - Mar 1985 UP 429/S6 were first assigned to Pocatello, then to East Los Angeles until March 1985.
  SD40 3030 May 1985 - Aug 1989 Carbody cut down. UP 3030/S6 were used at Pocatello until S6 was retired.

UP Twelve-Motor Slug Sets

Power Unit
S7 SD24 405 Apr 1978 - Apr 1981 UP 405/S7 were first assigned to North Platte, then to Hinkle???
  SD40 3034 Apr 1981 - Jun 1983 UP 3034/S7 were assigned to North Plate.
  SD40 3032 Jun 1983 - Jul 1989 UP 3032/S7 were assigned to East Los Angeles until UP S7 was retired.
S8 SD24 419 Dec 1978 - Apr 1981 UP 419/S8 were first assigned to North Platte.
  SD40 3035 Apr 1981 - Jun 1983 UP 3035/S8 were assigned to North Platte.
  SD40 3034 May 1984 - Apr 1985 UP 3034/S8 were assigned to North Platte until UP S8 was retired.