Locomotive Trucks

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This page was last updated on October 12, 2011.

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Scott Chatfield wrote on the Diesel List on April 14, 2002:

Most locos have two trucks. There are 4-axle locos, 6-axle locos, and even 8-axle locos. 2-axle and 3-axle locos have one truck.

To wit, quoting the 1974 "Car & Locomotive Cyclopedia":

"Truck: The general term covering the assembly of parts comprising the structures which support a car body AT EACH END [emphasis mine], (or, in the case of articulated cars, the joint support of two abutting car ends) and also provide for attachment of wheels and axles."

"Truck frame: A structure made of cast steel in one piece, to which the journal boxes or pedestals, springs and other parts are attached, and which forms the skeleton of a truck. See Truck Side Frame."

"Truck Side Frame: The longitudinal portion of a truck structure, on the outside of the wheels, which extends from one axle to the other, and to which the journal boxes and bolsters or transoms at attatched or form a part."

It's pretty clear that Demetre's understanding of a "truck" is correct. And industry references and advertisements generally call out the number of wheels a truck has, not its number of axles, if they bother calling it out at all.

James Mischke wrote on July 23, 2007:

The practice of naming trucks after their designers seemed common in the trade with the other builders early on (i.e. Batz, Blunt .... according to John Kirkland). It was fomented by the first Diesel's Spotters Guide by Pinkepank (1967), who needed to name hardware for brevity, with industry contacts and documentation scarce to nonexistent.

On the subject of what railfans commonly call the "AAR Type A" and "AAR type B" trucks. Researching in AAR proceedings and standards, I have never found any such AAR truck designation. I did find two locomotive wheel profiles, termed type A and type B, only remotely related to the trucks in question. This makes me doubt AAR involvement. From your knowledge, were there such things as AAR Type A&B trucks? What did the industry call those designs? I believe they were Commonwealth designs (whose name is commonly given only to a three axle truck Baldwin and FM used). Why did the builders gravitate to those designs? (Posted to LocoNotes, July 23, 2007 by James Mischke)

GSC "Type A" and "Type B"

Four-Wheel Switcher Truck "AAR Type A"

Four-Wheel Swing Bolster Truck "AAR Type B"

With the changeover from pre-World War II S-1 and S-2 production of switchers, to the post-war S-3 and S-4 models, "Alco discontinued the use of the Blunt ... and substituted in its place a drop side equalizer, rigid bolster cast steel design being used by Baldwin, EMD, Fairbanks­Morse, General Electric, Lima-Hamilton and others to the extent that it had practically become an industry standard. Quoting further from John Kirkland in his The Diesel Builders, Volume Two, American Locomotive Co., on page 97:

The Mechanical Section of the AAR had long campaigned for Alco to adopt this truck, first produced by the Commonwealth Steel Company of East St. Louis, Illinois, and later acquired by the General Steel Castings Corporation and marketed bearing the GSC trademark shield insignia. The patents on this truck had long since expired at this time, and several foundries were duplicating this truck with such exactness that its component parts were duplicate and interchangeable. Today, EMD is the major supplier of renewal parts for these trucks.

The 1947, 1948, and 1949 Annual Reports of the Association of American Railroads Committee on Locomotive Construction document the continuing deliberations which resulted in the adoption of the GSC switcher truck as an "AAR Recommended Practice" for all locomotive builders to follow notwithstanding Alco's protestations. This move forced Alco to change from the Blunt to the GSC switcher truck, for had this change not been made, Alco could no longer have sold switchers as being in conformance with AAR standards, and in turn acceptable for use by common carrier railroads.

This truck change in Alco's production was identified by designating the 660 hp switcher as model S-3, specification 1530A and the 1,000 hp switcher as S-4, specification 1540A.

Many railfan writers refer to the GSC rigid bolster switcher truck as AAR type A, and to the GSC swing bolster version of this truck as used under locomotives intended for road service, as AAR type B. The term "AAR Truck" appears to have originated with Alco's personnel at Schenectady, as a means of simplistically differentiating between the Blunt and the GSC rigid bolster switcher truck in the course of everyday communications. This is evidenced by handwritten entries made in Alco's Numerical Record that identify every locomotive manufactured in the United States and Canada and by its licensees in foreign countries. Handprinted notations appear sporadically in the Numerical Record, beginning with c/n 78140 for the model S- 3s and with c/n 78008 for the model S-4s, reading "AAR Trk" and marking the point where this truck change was first made. The same notations are then randomly entered in the production that followed. It was standard practice to build switchers on stock orders and to sell them out of stock as orders were received. In phasing out the Blunt truck, and phasing in the GSC truck, it was necessary to match Blunt trucks to orders received from customers already owning switchers equipped with Blunt trucks, so that interchangeability problems would not be created as would have existed had the transition point been arbitrarily made at c/n's 78008 and 78140.

At no point in the Numerical Record can an entry be found reading "AAR Trk" against road switchers or road locomotives built with a B-B wheel arrangement, since these units had been equipped with GSC swing bolster trucks that were acceptable to the AAR.

That the term AAR Truck did not originate with the AAR has been confirmed by a member of the AAR committee that voted to adopt the GSC truck as a Recommended Practice in 1949. Drawings for the GSC design switcher truck that appear in Section F of the AAR Manual of Standards and Recommended Practice are devoid of references to it as an AAR Truck, let alone as a type A or B. This would appear to leave the origin of this terminology to the devotees of railfan slang, whose fertile minds have generated such descriptive gems as Alligator, Baby Face, Centipede, Covered Wagon, Flying Box Car, and Shark to identify locomotive types.

It is conceded that the designations Type A and Type B provide a simplistic means for identifying these two truck types; however, since the AAR had nothing to do with their origin, it would be appropriate to identify these trucks in railroad reference books as GSC type A and GSC type B, in conformity with the industry practice of identifying the product by the name of its originator, as exemplified in the instance of truck types now carrying the names of ASF, Batz, Blomberg, Blunt, Dofasco, Flxible, Standard, Scullin and Symington, to name only a few.

On page 75, while describing the features of the newly designed 1000 horsepower road switcher later known as the RS1, Kirkland writes:

To provide improved riding characteristics (so as to impose minimum stress on lightly constructed track) these locomotives were built with a B-B wheel arrangement, using drop side equalizer, swing bolster trucks. These trucks had a rigid wheelbase of 9' 4", compared to 8' 0" for the standard switcher. This longer wheelbase truck distributed the weight over a longer rail distance (involving more tie support) which benefited operation over branch line track maintained to substandard conditions.

On page 118, while describing the features of what later became known as Alco's FA/FB locomotives, Kirkland calls the four-wheel freight truck, "Conventional two axle swing bolster, drop side equalizer trucks with 9'4" wheel base and 40" wheels". Also calling them "B-B drop side equalizer rigid bolster trucks with 9'4" wheel base".

Kirkland makes no mention of the three-axle truck used under the DL series, or the PA/PB series locomotives, other than to describe them as either 15'4" or 15'6", ridgid wheelbase A1A trucks.

Posted to LocoNotes, March 29, 2000 by James Mischke:

Was the familiar AAR Type A switcher truck and AAR Type B road truck really designs from the Association of American Railroads? My hypothesis is that they were not.

  • I can not find any AAR reference to that nomenclature. Neither can a good friend of mine at the AAR headquarters in Washington.
  • The first mention of "AAR Type A" and "AAR Type B" trucks seems to be the original 1967 Diesel Spotters Guide. Both DSG authors have vigorously assured me in person that the AAR designation is valid, but could cite no documentation when pressed.

My recommendation would be that we continue to call the trucks in question "Type A" and "Type B" (have to call them something, and everyone knows this nomenclature) but drop the AAR adjective.

Posted to LocoNotes, April 3, 2000 by David Thompson:

In his Alco book, Kirkland mentions that the designs were adopted by the AAR as a "Recommended Practice" in 1949, even though they had existed since the mid-1930s in some form. The "Type A, Type B" description seems to have originated with the Diesel Spotters Guide. Kirkland suggests calling the two designs General Steel Castings (GSC) Type A and B.

Does anyone happen to know how Commonwealth came to be GSC? (I'm guessing a Depression-motivated consolidation in the early 1930s)...

Posted to LocoNotes, July 24, 2007 by Scott Chatfield:

"Commonwealth" was a brand name of General Steel Castings, a foundry co-owned by Alco and Baldwin. The brand name was used on just about everything GSC made, not just trucks. So there were Commonwealth steam loco frames, Commonwealth tender trucks, Commonwealth passenger car trucks.....you get my drift. So the term is rather non-specific, and calling a Type B road switcher truck a "Commonwealth" is almost pointless. For that matter, GSC also cast "Blomberg" truck sideframes for EMD, but you won't hear me calling any Geep truck a "Commonwealth."

I think linking a designers name to a truck design is a good thing and would eliminate most of the confusion we've seen for the last, what, 40 years regarding what to call the standard drop-equalized road switcher truck found on most 4-axle diesels not built by EMD before 1970. Unfortunately, the identity of that truck's designer has been lost to history. And just as unfortunately, 40 years of calling it an "AAR Type B" will be difficult to erase from the collective memory of railfans.

"Type B truck" is easier to say and type than "Commonwealth drop-equalized road switcher truck." And if everyone understands what it means, it isn't a bad a thing. After all, despite my repeated threats of physical violence, too many posters still call plain journal bearings "friction bearings." There is such a thing as a friction bearing in the railroad biz, but it's not a _journal_ bearing. But this inaccurate usage will last just as long as "AAR Type A" and "AAR Type B" to describe loco trucks. We'll just have to live with it.

For the four-wheel freight truck, Craig Rutherford wrote on LocoNotes on July 2, 2002:

To expand just a little further on Jim's point, I quote from the Alco-GE Locomotive Specifications for the 1600 HP A unit, Section 3-Trucks :

TYPE - Two four-wheel, swivel, swing motion, pedestal type motor trucks applied.

CONSTRUCTION - Cast steel construction. Frame is spring supported on two equalizers on each side, with triple coil springs between the equalizers and the frame. Triple elliptic springs are applied between bolster and spring plank. Center plate safety locks applied.

AXLES - Two (2) axles, of forged open hearth steel, per truck.

WHEELS - Four (4) rolled wheels, 40" diameter, per truck, to A.A.R Specifiication M-107-48, Class "B".

Posted to LocoNotes, July 23, 2007 by "boogieengineer":

My colleagues at EMD and I simply referred to these trucks as drop equalizer trucks either rigid or with swinghanger secondary suspension.

EMD Four-Wheel "Blomberg" Truck

Posted to LocoNotes, July 22, 2007 by "bogieengineer":

Call it a Blomberg truck at EMD and most engineers would not know what you're talking about. There were two versions of the GP swinghanger truck to EMD - the GP single shoe and the GP clasp brake truck. Anything else is just a modification of one of the those. Americast, formerly Atchison, Rockwell, and LFM calls it the Freight truck. If railfans insist on naming trucks after their designers, then the HTC should be the Mels truck, the SD Flexicoil the Sundby truck, the HTB the Garg truck, and the GE Steerable truck the McGrew truck . You'd also have to name a truck for me.

EMD Four-Wheel Switcher Truck

Posted to LocoNotes, July 22, 2007 by "bogieengineer", in response to a question about the difficulties of swapping an AAR switcher truck and an EMD Blomberg truck:

If by AAR switcher truck you mean the EMD switcher truck used on most EMD switchers that has no secondary suspension, there are many differences. Most significant are the wheelbase is 8' on the switcher truck and 9' on the GP swinghanger truck and the system on the switcher truck where the traction motor air passes thru the centerbearing into the truck frame then to the motors. I believe the underframe height is less on the switcher due to the short suspension travel so coupler pockets get affected. But the centerbearing would definitely have to change as well as creating an air plenum and air duct openings in the underframe.

EMD High Adhension HT-B Truck

Posted to LocoNotes, July 18, 2007 by "Joseph":

In December 1977, EMD introduced the optional [experimental] high-adhesion HT-B truck. The HT-B truck was used on ten GP40X testbed locomotives built for Southern Pacific and Union Pacific [SP 7200-7201, 7230-7231/UP 9000-9005].

The remaining GP40X testbeds [Atchison, Topeka & Santa Fe 3800-3809/Southern Railway 7000-7002] rode on Blomberg-M trucks.

Was the experimental HT-B truck designed around the new EMD single-axle, Super Series wheel-slip detection control system? The new wheel-slip system utilized 'Doppler-type' radar signals to determine ground speed. Was this wheel-slip detection/correction system and/or the HT-B design itself borne out of earlier cooperation between EMD and ASEA in the 1970's?

I believe the design of the HT-B truck was to minimize weight transfer between the axles due to the torque created by the application of higher horsepower to maximize adhesion.

In addition to the obvious truck frame design changes, the only differences I have noted is the use of 42" wheels [instead of 40"] and an approximate one inch height difference [HT-B vs. Blomberg-M].

My understanding is that the HT-B was offered as an option on the EMD GP50. I believe that railroad motive power buyers/managers of the time were hesitant to purchase then unproven technology. Stocking of additional parts was probably another issue in the decision-making process. I've read that the HT-B truck suffered from ride-quality/tracking [hunting] issues.

I have no experience riding any of the HT-B equipped locomotives, but I will say that most Blomberg trucked locomotives ride and track well at high and low speeds.

From Scott Chatfield on LocoNotes on July 20, 2007:

It was listed as an option, but the reality is the HT-B would not fit under the standard GP50 frame. Keep in mind that the HT-B was somewhat longer, with a 9' 9-3/4" wheelbase, and had a lower bolster. The GP40Xs were all built with a one-foot longer frame, mainly to clear the HT-B. That EMD reverted to the GP40-2 frame for the GP50 is evidence (to me, at least) that by mid-1980 they had given up on the HT-B. I suppose that if a road like Santa Fe which got smaller fuel tanks on their GP40Xs and GP50s had also ordered HT-Bs, EMD may not have used the longer frame.

The HT-B was a rough rider because of the rubber/metal sandwich that formed the primary suspension. The Blomberg-M had the same problem, which is why EMD went back to leaf springs by the late '80s.

The lower bolster was designed to reduce the tendency of the axles to twist about the axis of the bolster, because the traction motors are pointing in opposite directions.

Robb Fisher wrote on LocoNotes on July 20, 2007:

The GP40Xs were built with a longer frame to accomodate a standard fuel tank along with the HT-B... recall that a GP40X has a 1' longer wheelbase, negating any clearance you'd gain with a 1' longer frame.

On the GP50, EMD spec'ed smaller tanks (3400g) for models optioned with HT-Bs... the other models mentioned had 2600g tanks standard, so clearance would not have been an issue.

Also, EMD did NOT revert to the GP40-2 frame for the GP50. They went back to the same length, but it is not the same frame as a -2.

"Bogieengineer" wrote on LocoNotes on July 20, 2007 :

The HT-B truck was conceived at EMD in the early 1970's to address several issues:

- Poor weight shift performance of the GP swinghanger truck under high adhesion conditions
- Reduced vertical ride quality of the GP single shoe truck introduced with the rubber compression pad that replaced the elliptic springs,
- Lower maintenance costs and extended intervals by eliminating spring plank bearing blocks, swinghanger pins and bushings, and bolster chafing plates.

To address the weight shift performance, the inclined rubber chevron springs that focused at the top of rail were developed. This arrangement essentially eliminates weight shift within the truck so that only carbody weight shift remains.

To improve the vertical ride, the long travel primary springs used on the HT-C truck were incorporated. That is why the truck is taller than the GP truck. Unfortunately, elimination of the swinghanger lateral suspension degraded the lateral ride, which I am sure would have been overcome with changes to the rubber secondary spring chevron angle, which sets the secondary lateral stiffness.

The maintenance wearing parts are reduced to the pedestal liners and journal box wearplates thru use of the rubber secondary suspension.

I have no doubt the truck would have been a success if it was introduced as a running change in the Dash 2 series. The GP40-2 with its WS10 correcting wheelslip system could have made good use of the lower weight shift. The introduction of the HT-B with the GP40X and the SuperSeries wheelslip control,and particularly with some of those equipped with GP swinghanger trucks, provided the opportunity for head-to-head testing of locomotive adhesion performance. The control engineers could not measure an appreciable difference in overall locomotive adhesion between GP40X units with the different trucks. This was attributed to the increased, but controlled, slip at the leading axle of the GP truck which had a beneficial cleaning effect allowing the other axles to pull harder and make up the difference.

And, of course, what really killed it at EMD was the increased cost to manufacture. Although the design is quite simple and it eliminates many parts, the fact it is new means all the parts cost more than mature parts, and the GP truck could not have been more mature. The underframe was more costly because of a different centerbearing arrangement. While there are documents to show that it was offered on models beyond the GP40X, it was a dead item within EMD shortly after the GP40X's came out.

It is not true that it is "interchangeable" with the GP truck but that depends on the definition used. The HT-B has a much larger diameter centerbearing that is mounted on structure that hangs down from the underframe about 24". When units were converted to GP trucks, this structure had to be cut off and a conventional GP centerbearing welded in place. I hope the shops that did it knew what reinforcements are required above the bottom plate to react the centerbearing. Because the whole underframe sits higher, the coupler pockets on the HT-B units had to be longer to put the coupler at the proper height. The traction motor cabling was also routed differently and the longer wheelbase means the traction motor air ducts have to move. It's certainly possible to modify a unit to take one truck or the other but it is not trivial.

EMD's Six-Wheel Flexicoil Truck

Union Pacific's SD45s all had double clasp brakes for the additional braking capacity. These units all had cast iron brakes shoes, for the same reason. These trucks carried EMD's "Flexicoil" brand name and were externally identical to the trucks used on UP's earlier SD7 and SD24 locomotives, although the SD45 trucks differ from the earlier SD7 and SD24 trucks in the location of the traction motor air ducts. The newer Flexicoil trucks that were available in 1966 on the first SD40s and SD45s for other railroads were not available with double clasp brake shoes, so EMD was forced to use its older design to fulfill UP's needs, and all of UP SD45s were delivered with these trucks. By mid to late 1968, EMD was able to modify its newer design with double clasp brakes, which were selected by other railroads, such as the N&W. Later, several UP SD45s were equipped with this later double clasp brake truck, including some units with one truck of each type. Because the earlier design was no longer in production by EMD after 1968, these later, different trucks were the result of truck replacement through normal maintenance by Union Pacific, since the two designs are functionally identical.

Bob Campbell wrote on the Burlington List on January 11, 2003:

The "Q" used clasp-type brakes with cast iron brake shoes (two per wheel) on all their diesels. Your model should have six 8"x 8" brake cylinders per truck. They were really good engine brakes that operated on 45 psi air pressure, unlike the single (composition) brake shoe per wheel post-merger units that operated on 72 psi air pressure. The 516-530 were delivered early in 1969 three years after the GN got the first production SD-45 #400 "Hustle Muscle" in 1966. I believe the BN received the last SD-45 built, 6567 in 1971.

Terry LaFrance wrote on the Burlington List on January 13, 2003:

Don't confuse truck design with brake set up. The SD35 and earlier trucks are not NECESSARILY the same as the SD40/45 trucks, some are some aren't.

Produced in several versions, EMD's Flexicoil truck featured roller bearings and had a 13 foot 7 inch wheelbase. It was available in the following phases:

Bill Barber wrote on the Burlington List on January 11, 2003:

The trucks under SD40/45s were a revised design different from those used on SD7/9s. The original SD40/45 trucks (1966) had four large (about 11 inches) brake cylinders mounted under the truck frame between the wheels. These brake systems were single shoe per wheel and used composition brake shoes, among the first such applications on locomotives Earlier SDs used traditional clasp brakes with two cast iron shoes per wheel and six brake cylinders. Independent brake cylinder pressure was raised from 45 psi to 72 on the single shoe application. The braking characteristics of composition shoes are considerably different from cast iron shoes. Crews had difficulty getting used to them. They didn't brake as well, particularly at slow speeds. The original underslung mounting arrangement was troublesome and relatively high maintenance. In addition, there was a problem with the hand brake. It could be set against the air in the brake cylinder which if it bled off would allow the hand brake to release.

By the time that the Q ordered their SD45's in 1968, these negative conditions were rather well known. The Q and a few other RRs, (notably Norfolk and Western) specified a traditional clasp brake cast iron shoe arrangement on their locomotives. NP and GN both had very early SD45s and continued to ordered the original arrangement, which, by 1969, was modified. The first modification was three cylinders low and one high at one corner of the truck. The hand brake was connected to the lever of the high cylinder, eliminating the bleed off problem. As further problems occurred with the low slung cylinders, all four were eventually raised to the high corner positions. In 1972, the truck was totally superseded with the high adhesion Dash 2 truck. These were also available with single shoe or the clasp brake arrangement, but all brake cylinders were always high mounted.

Scott Chatfield wrote on the Diesel List on April 14, 2002:

There are six basic types of >> 3-axle << Flexicoils:

The Flex-C1 is the truck used on SD7s, SD9s, SD18s, SD24s, and early SD28s and SD35s. Starting around September 1965, EMD introduced the Flex-C2L to production units. (It's first use was under the first SD45 built in 2/65, but remember the SD45 didn't go into production until 5/66.) Some customers had trouble with the low cylinders getting banged up in minor derailments, and the connection to the handbrake didn't work very well, so the "high-low" version was introduced around February 1967. The original Flexicoil-C2 castings could not accomodate any more than one high brake cylinder. Moreover, if the customer wanted clasp brakes, the Flex-C2L could not accomodate them so the units were delivered on Flex-C1s. UP's SD45s for example. In 1968 EMD redesigned the Flexicoil-C2 castings (made the bolster slightly wider, for one thing) so it could use high cylinders all around, and other sideframe casting was made for clasp brakes. So later SD38s, SD40s, and SD45s delivered with clasp brakes have the Flex-C2C.

Amtrak's SDP40F Truck

Rob Sarbarenyi wrote on the Espee List on September 2, 2007:

Several derailments occurred involving Amtrak EMD SDP40Fs operating on the Burlington Northern, Louisville & Nashville, and the Seaboard Coast Line. These accidents led to their restricted use on those roads, while other railroads issued some limitations on the units. Southern Pacific didn't experience the same issues with SDP40Fs operating over their rails, but nevertheless, did initiate and take a few precautionary measures.

Amtrak and the Federal Railroad Administration (FRA) conducted comprehensive testing to determine the problem. EMD's SDP40F design was largely based on well-proven SD40-2 technology, although the units were longer and considerably heavier with their skid-mounted steam generator equipment, along with the necessity to carry water for the boilers. The SDP40F had a distinctive hollow bolster mounted HT-C truck, a modification from the standard HT-C trucks used on the SD40-2, SD45-2, and SD45T-2. FRA testing was unable to conclusively determine the faults of the derailments, and whether or not the truck's design was the culprit. Note: it's doubtful the trucks were the problem, hollow bolster or not, since thousands of freight units rose on similar trucks without these same issues. Further speculation deemed it possible that certain rail harmonics generated by baggage cars following the locomotives through the curves may have been contributing factors to the derailments. I don't believe a final determination was ever made.

Meanwhile, in the mid to late-1970s, Amtrak was already in the process of introducing the EMD F40PH into service, and thus decided to make these new units their standard on long distance passenger trains, in addition to shorter runs for which they were initially designed. EMD's SDP40F was phased out of Amtrak service, If I recall correctly, the last run took place sometime in 1984-85.

General Steel Castings Tri-Mount Six Wheel Truck

From John Kirkland's The Diesel Builders, American Locomotive Company, page 148:

Three axle trucks of conventional drop side equalizer rigid bolster design were supplied by General Steel Castings Corporation. These trucks provided a three-point support for the carbody, instead of the conventional single-point support by means of the truck center plate alone.

This new design was identified as a Tri-Mount type of truck. The conventional center plate constituted one support point. Pads positioned inboard from the center plate and mounted on the top surface of each truck side frame constituted the other two support points. These pads made a sliding frictional contact with mating surfaces on the underside of the locomotive underframe.

This three-point support system served to stabilize the truck. It minimized the normal weight transfer reaction that takes place from the leading to the trailing truck axles when a locomotive gets under way, and as a result, the factor of adhesion with the locomotive getting a train under way was increased from the normal 18 percent to a maximum of 22 percent. Additionally, this three point support system resulted in improved riding characteristics at road speeds.

The truck rigid wheel base was 12'6". With the traction motors hung inboard from the extreme axles, an irregular axle spacing of 5' 7" resulted between the middle and the inboard axle and of 6' 11" between the middle and the outboard axle. The truck's equalization system, incorporated in its spring rigging, provided uniform weight distribution on all axles.

The Tri-Mount trucks installed on the model RSD-4s were equipped with a single brake cylinder mounted on each truck side frame, which actuated a single shoe brake on each wheel.

More Information

Derrick Brashear's locomotive trucks web page -- There are a few photos of model versions, but not of the prototype versions.