Union Pacific SD24s

By Don Strack (January 1998)

(This article is an updated and expanded version of text used in the book "The GP20 and SD24, EMD's Turbocharged Duo" by Withers Publishing Co., 1998, ISBN 1-881411-19-2)

Union Pacific's SD24s were initially used, following their June through September 1959 delivery, on the railroad's South-Central District (the historic Los Angeles & Salt Lake Railroad), between Salt Lake City, Utah, and Los Angeles, through Las Vegas and southern Nevada. The arrival of the SD24s (30 cab units and 45 cabless booster units) augmented the large fleet of GP9s and GP9Bs. New SD24s also allowed UP to cut off helper districts on California's Cajon Pass and Cima Hill. The SD24s reigned supreme on the South-Central District in sets of six to eight units for about 10 years, until they were displaced in 1969 and 1970 with the delivery of UP's unique 6900-class Centennial locomotives, and were re-assigned to the general motive power pool. In 1971, four SD24s and four SD24Bs were assigned to U. S. Steel's Atlantic City (Wyo.) taconite iron ore unit train that operated between the Atlantic City mine in central Wyoming and U. S. Steel's Geneva (Utah) steel mill.

Three UP SD24s were rebuilt by the railroad in its constant search for improved motive power. UP 423 was rebuilt to the unique UP 3100 in 1968, and UP 410 was rebuilt with a modernized electrical cabinet and 645-design power assemblies in 1974. UP 410 received air intake modifications, making it similar to EMD locomotives built after the GP30.

When first rebuilt from UP 423 to UP 3100, the unit was equipped with a constant speed EMD 16-645 3000 HP engine (with speed control taking place by changing the level of excitation of the main generator) and traction motors permanently connected in parallel. UP 3100 was also the first unit on UP to have the capability of a self-load test, rather than being connected to an external load box. This self load feature was first delivered on UP's DDA40Xs, and on almost all production units from EMD after 1971. After 1975, UP 3100 (by this time renumbered to UP 3200, then 3399, then 3999, then finally UP 99) worked out its remaining days a heavy switcher in UP's North Platte, Nebraska, hump yards, and was scrapped in late 1985.

UP 445-447 were former EMD demonstrators, numbered as EMD 7200-7202 and painted in SP's new "bloody nose" paint scheme for testing on Southern Pacific - they even had SP's Gyralite nose mounted warning light. After their brief tour, UP picked them up at a reduced price.

UP 448 was the first SD24 built. EMD completed the unit as EMD 5579 (also its order number) in July 1958 as a test bed for their new turbocharged, 16-cylinder engine. In its earliest configuration, the unit included a radiator section over the turbocharger area. It was rebuilt to production SD24 appearance in August 1960 and sold to UP, becoming the road's only high hood SD24.

In their later days, most of the remaining UP SD24s continued on as heavy switchers (at times assigned as power units for the road's small fleet of yard slugs) in hump yards in North Platte, Nebraska, Hinkle, Oregon, and Los Angeles. Many cab units, and almost all cabless booster units, were retired in 1977. The last SD24 on UP was UP 446, retired in May 1985.

Qty Year
UP 400-429 30 Jun-Sep 1959 5602  
UP 400B-444B 45 Jun-Sep 1959 5602 No cabs
UP 445-447 3 Aug 1960 5623 To UP in Aug 1961
UP 448 1 Jul 1958 5579 To UP in Aug 1960

(Complete roster listing of UP's SD24 locomotives)