A-B-A: why two A units ?

[dan green]

Hi, I am wondering why the A-B-A diesel engine configuration uses a reversed cab unit and not a cabless B unit at the rear? This seems a standard configuration for Super Chief model sets, was is a standard configuration for the real life Super Chief?

[scottychaos]

simple..because the ABA set is reversable without having to turn it on a turntable or wye..

an ABA set has two "fronts"..it can lead in either direction..an AB set only has one front...you cant very well run a streamliner passenger train with a B-unit in the lead! ;)

Scot 

[IRONROOSTER]

With 2 A units facing opposite directions you don't need to turn them at the end of a run to go back.  Just run the engine to the other end.  Also, I think more A's were bought than B's which gives the railroad more flexibility since you can assign an A or AA to a train but I don't think the B's could run alone or at least not easily - need an A.

Enjoy

Paul 

[dan green]

Yes, that is simple! thanks for the clarification. It sounds like this configuration suited passenger services where an efficient turn around was required. Was the reversed cab used less on freight services or was it just as common?

 

[passengerfan]

Santa Fe generally ran A-B-B-A sets of F3 or F7s on the Super Chief and El Capitan and the postwar Chief ran A-B-A sets of Alco PAs or initially it was not unusual to see A-B-B-A sets of FTs.

Later sometimes a fifth unit was added to the El Capitan and Super Chief either an A or B unit it did not seem to matter.

Only the EMD B units came with steam generators, the Alcos had steam generators in both the A and B units.

Al - in - Stockton  

[Texas Chief]

Al has it correct. Santa Fe rarely, if ever, ran only one FB unit, and never ran more than one PB unit. Both F units and P units were used on the Express fast mail also, but never together.

Dick

Texas Chief

[wjstix]

The first F units, the FT's, were originally designed to run in A-B sets with a drawbar between the two, so you could run A-B or A-B+B-A. Many railroads found that one A-B set wasn't enough power for a typical train, and two were too much, so after WW2 many of them bought single F2 (or F3) A units to make FTA-FTB-F2 sets which (like the Baby Bear's porridge) were 'just right'.

One other advantage of having A units as both ends (besides not having to turn them at the end of a run) was for switching / moving the engines. A good example was the Northern Pacific who used A-B-B sets on the North Coast Limited in the forties. In St. Paul MN the NCL from Chicago would be backed into St.Paul Union Depot by the CB&Q who ran the train from Chicago to St.Paul, and then NP engines would have to back onto the train to begin pulling it the rest of the way to the west coast. The engineer sat on the right side of the A unit of course, and had to try to back the engines down a curve to the left, so was basically blind trying to do it, and had to rely on information relayed by the fireman. Eventually they bought additional A units to make A-B-A sets, at least in part so that the engineer could just walk to the other end of the A-B-A lash-up and run the engines from there with a good view of the situation while backing up, then could walk back to the other A unit to head out with the train.

BTW if you look at trains today, you'll notice that most lash-ups of two or more diesels still will have the 'outside' engines facing away from each other, so that they can be run as a set in either direction without needing to be turned...so if you have say two SD-70's running together, likely as not the two engines will be back to back, or if there were three engines, the 1st and 3rd ones would be facing away from each other.

[daveklepper]

The FT's were the only cab diesels of EMD that lacked space for boilers in A units.  (Possibly the interem F2, as well.)  Small boilers could be fitted into F3 A-units, and some did have them.  With F7's, the FP-7 was the boiler equipped type and FP-7 A units did have boilers.   FL-9's originally had boilers, also, but all surviving on Metro North or Amtrak were rebuilt for headend power.   There were no FL-9 or FP-7 B units.

The B&M originally bought A-B-B-A sets of FT's following the barnstorming demonstration.   They learned two things:  One and one B would be ideal for some passenger runs, so some B units were quickly equipped with boilers and AB sets used for passenger service.    Also, they found that most of their freights could be handled with only three units.  So they bought F-2 and F-3 single A units to make ABA combinations to fit their frieght purposes.

Double-end passenger power on the B&M wasn't critical, since most passenger runs started and ended and major terminals with turntables and/or wyes.   But freights often did pick-up and set-outs, often required reverse moves to reach branchline points, and double end power was a great improvement over steam operations, so nearly all freights ran with an A-units on each end, even AA combinations.   Of course, the B&M was an early convert to road switches, with Alco's RS-types at first and then GP-7's. and many freight ran with a mixture of road-switchers and B and A cab units while they lasted.

E units were rarely used in multiple, but mostly handled the flat Boston - Portland and Boston - Poprtsmouth runs where one E-unit usually had enough power.

[Kurn]

Not to throw a wrench into the works,but there was Rock Island's AB6 units......

[passengerfan]

Not only did Rock Island have the AB6s that were E6B units with a a cab built into one of the blunt ends a single EMD 1000 hp power plant and baggage compartment , but the MP had AA which was an E6A unit with a single power plant and baggage compartment. The CB&Q had the Silver Charger the last shovelnose built for the railroad was essentially the same as MP AA.

Al - in - Stockton   

[Kevin C. Smith]

Not to throw a wrench into the works,but there was Rock Island's AB6 units......

Which always had me wondering...why not just mu two A units? Why add a cab to a B unit for a "one off" oddball locomotive type?

[passengerfan]

Many railroads operated E units particularly back to back such as the Great Northern initially with the new 1947 Empire Builders. The 4000 hp with the A-1-A trucks were slippery in the climb over Marias Pass and GN went to A-B-A sets of F-units later A-B-B-A F-units with the introduction of the Mid-Century Empire Builder.

The Santa Fe purchased many prewar E-units and did away with them except on connecting runs and short distance trains with the one exception of the Chicagoan and Kansas Cityan but the operational territory of this train was relatively flat. Even this train eventually received F-units and PA units. By 1948 the E units were gone from the long distance trains altogether on the Santa Fe.

The CB&Q another example utilized E units for the most part except for purchasing three A-B-A sets of F3s as the initial power for the California Zephyr. By 1951 even these F unit were gone from Passenger service and regeared for freight. The CB&Q only purchased a few of the E5 B units so on the CB&Q it was not unusual to see AB sets of these on the head of many trains. The CB&Q operated as many as three E units on trains such as the California Zephyr and Denver Zephyr and it was standard practice for all three E units to face the same way (elephant style). The reason they faced the same way was there was adequate turning facilities in Denver and the E units were in a pool that also operated Chicago commuter trains from Union Station to Aurora. Any E road unit could be found in this service except for the E5 units with rare exception.

Al - in - Stockton 

[daveklepper]

The Rock Island's two E6 B-unis with blunt-end cabs were designed for a specific purpose for specific train:  The Rocky Mountain Rocket.   This train had both units to Limon, Kansas, then the train was split with half going to Denver (mostly UP trackage rights) and half going to Colorado Springs.   The blunt-end B-unit kept the streamlining of the train intact from Chicago to Limon, then could pull the Colorado Springs section from Limon, while the A-unit went to Denver.   Going east the reverse procedure took place.   This was a fully streamlined train with round-end observation, and so the esthetics were important.   The B-unit cab s of course wound up in Chicago suburban service where they mixed with a whole variety of diesels, usually running solo.

[wjstix]

The Rock Island's two E6 B-unis with blunt-end cabs were designed for a specific purpose for specific train:  The Rocky Mountain Rocket.   This train had both units to Limon, Kansas, then the train was split with half going to Denver (mostly UP trackage rights) and half going to Colorado Springs.   The blunt-end B-unit kept the streamlining of the train intact from Chicago to Limon, then could pull the Colorado Springs section from Limon, while the A-unit went to Denver.   Going east the reverse procedure took place.   This was a fully streamlined train with round-end observation, and so the esthetics were important.   The B-unit cab s of course wound up in Chicago suburban service where they mixed with a whole variety of diesels, usually running solo.

Kalmbach has published an interesting series of photos of the two trains being joined together in MR and possibly Trains, plus in a couple of model railroad books and special issues...I think it's in one of the Armstrong track planning books for example, but I can't recall right now which one.

[Autobus Prime]

The FT's were the only cab diesels of EMD that lacked space for boilers in A units.  (Possibly the interem F2, as well.)  Small boilers could be fitted into F3 A-units, and some did have them.  With F7's, the FP-7 was the boiler equipped type and FP-7 A units did have boilers

dk:

F7 and F9 A units could be fitted with heating boilers, too. The gewgaws on the rearmost hatch, if present, are the vents and fillers for this. They only held about 800 gallons of water, though, where the FP7s held twice that.

[wjstix]

On the North Coast Limited, the NP had baggage cars that were set up with large water tanks in the 'front' (the part closest to the engines) with piping set up so they could provide water to the F unit's steam generators. IIRC Minnesota Transportation Museum has one of these that they use in trains on their Osceola and St.Croix excursion trains.

BTW as I mentioned earlier they usually used A-B-A sets of F's on the trains, but when going west often added another engine when going thru the mountains, so you'd have an A-A-B-A consist out front.

[THE.RR]

BTW if you look at trains today, you'll notice that most lash-ups of two or more diesels still will have the 'outside' engines facing away from each other, so that they can be run as a set in either direction without needing to be turned...

That may be true of the head end set, but the DP on the rear can be any combo.  I quite often see 2 units nose to nose on the rear.  Looks odd, but since there is no crew on board, it really dosen't matter.  Just grab the next 2 units off the ready track and tack them on. 

Phil

[THE.RR]

[...why not just mu two A units?

What DaveKlepper said, plus 99.44% of F's, E's, PA's, etc were BUILT with out MU connections on the nose, so operating elephant style was not possible.  The railroads later rectified this problem. 

Phil

[wjstix]

[...why not just mu two A units?

What DaveKlepper said, plus 99.44% of F's, E's, PA's, etc were BUILT with out MU connections on the nose, so operating elephant style was not possible.  The railroads later rectified this problem. 

Phil

Soo Line would be a good example, they never added m.u. connections to the noses of their F units, so even into the 1970's you could see an A-B-B-A lash-up of white red and black SOO F's together...or a couple of F's with a GP stuck in between!!

[passengerfan]

On the North Coast Limited, the NP had baggage cars that were set up with large water tanks in the 'front' (the part closest to the engines) with piping set up so they could provide water to the F unit's steam generators. IIRC Minnesota Transportation Museum has one of these that they use in trains on their Osceola and St.Croix excursion trains.

BTW as I mentioned earlier they usually used A-B-A sets of F's on the trains, but when going west often added another engine when going thru the mountains, so you'd have an A-A-B-A consist out front.

The reason the NP operated the water baggage cars on the North Coast Limited can be found going back to steam locomotive days. The NP bad water district stretched from eastern Montana through the western half of North Dakota. In Steam days water was hauled into this area for the locomotives to use and the NP did not need the associated problems with the diesels steam generators thus the water baggage cars on the North Coast Limited from the beginning of the postwar diesel age. The water baggage cars were able to carry enough water that none was needed through the bad water area. If just the water carried by the diesels they would not have had enough to get through the bad water district.  

Al - in - Stockton

[Packers#1]

simple..because the ABA set is reversable without having to turn it on a turntable or wye..

an ABA set has two "fronts"..it can lead in either direction..an AB set only has one front...you cant very well run a streamliner passenger train with a B-unit in the lead! ;)

Scot 

exactly what I was going to say. With an ABB, you have to turn around for the cab to be on the front. With an ABA, the crew just has to go to the other cab to swap ends.

[Lyon_Wonder]

Interestingly, while over one third of F unit production were cabless B models, very few cabless GP7s and 9s were built.  Though UP ordered GP30s without cabs, and much later ATSF with the GP60Bs, road switchers offered the flexibility that was hard to get with cab body F-units. 

[Denver Zephyr]

The Colorado & Southern(C&S) and Fort Worth & Denver(FW&D) ran A-B-B-A sets normally and would put an SD (or 2) between the B's if more power was needed.  Also AA or ABA sets on passenger trains can be serviced while a switcher is wyeing the train, thus saving terminal time. Time always computes to money.  At Minneapolis during the late 40's, the Twin Zephyr would arrive with a E-5A and a Shovel Nose back to back.  While the train was(cleaned) wyed the units would be serviced and return with the E-5A leading on the return trip(all that in 45 minutes).  This only aplied to the Morning TC Zephyr in and Afternoon TC Zephyr out.  Don't know how it was done in Chicago but the time factor was simular.  The track layout in Chicago was more complicated with other railroads and trains in the mix.  

[Denver Zephyr]

Just a interesting note about those NP baggage cars w/water tanks.  In 1977 AMTRAK did somekind of trade and became owner of several of these cars.  They still had the water tanks.  AMTRAK used them for mail on the rear of 9 & 10 CHI-MSP-CHI.  7 years after BN merger and 6 years after AMTRAK, they still exhisted and still in NP Paint with tanks in tact, still had the needed piping.

[Lyon_Wonder]

Not only did Rock Island have the AB6s that were E6B units with a a cab built into one of the blunt ends a single EMD 1000 hp power plant and baggage compartment , but the MP had AA which was an E6A unit with a single power plant and baggage compartment. The CB&Q had the Silver Charger the last shovelnose built for the railroad was essentially the same as MP AA.

Al - in - Stockton   

I assume the single 567 was still a V12, though I suppose EMD could have upgraded it to a V16 if the railroad asked?

[CSSHEGEWISCH]

Not only did Rock Island have the AB6s that were E6B units with a a cab built into one of the blunt ends a single EMD 1000 hp power plant and baggage compartment , but the MP had AA which was an E6A unit with a single power plant and baggage compartment. The CB&Q had the Silver Charger the last shovelnose built for the railroad was essentially the same as MP AA.

Al - in - Stockton   

I assume the single 567 was still a V12, though I suppose EMD could have upgraded it to a V16 if the railroad asked?

The AB6's were originally equipped with a single V12 567 engine, same as the AA6 (MP 7100).  RI later put a second 567 in the baggage compartment, making it the equivalent of a conventional E6A.  They were later equipped with a diesel-alternator HEP set when RI bought its first bi-level coaches for suburban service.

[Denver Zephyr]

The Rock Island AB6 units were originally E-6B's.  Rear prime mover removed and baggage area created.  When they became comuter power the Rock Island reinstalled the prime mover.   When the R.I. acquired the Aerotrains they operated with a LWT-12 on each end of one set(A-train-A), half the cars faced one end and half faced the other.

[spikejones52002]

Denver Zepher, When did the Rock run a pull - pull unit like you describe?

I use to watch the Aero train pass every week day on 103th and Vincennes as I went to school.

Do you have any photos?

The only differance I seen was that train unit #3  was not articulated cars.

[jeffhergert]

BTW if you look at trains today, you'll notice that most lash-ups of two or more diesels still will have the 'outside' engines facing away from each other, so that they can be run as a set in either direction without needing to be turned...so if you have say two SD-70's running together, likely as not the two engines will be back to back, or if there were three engines, the 1st and 3rd ones would be facing away from each other.

On most railroads, especially the class 1s, the direction the engines are facing behind the leader is a coincidence.    

The exceptions are for trains that will terminate or "turn" at a location where there are no turning facilities or they are inconvenient to use.  Then an effort is made to have an engine facing the other direction, but even then it may not be the outer most engine.  

Jeff

   

[passengerfan]

When the NP dieselized the North Coast Limited originally they assigned A-B-B sets of F3s to keep the postwar train esthetically pleasing to the eye. They operated the trains in this fashion between St. Paul and Seattle. Since the wye in Seattle was some five miles south of King St. Station where they turned the train, the RR soon broke up the A-B-A sets with a F7A replacing the one B unit. This gave the NP A-B-A sets of passenger power that became the standard for the North Coast Limited. These sets operated Livingston - St. Paul - Seattle - Portland - Seattle - Livingston before being changed. They were always changed eastbound at Livingston. If a fourth unit was required during heavy summer traffic it was always an extra A unit. The only exception in the west on NP was the power for the Seattle - Spokane train that most times operated with an A-B set of F units. This train rarely exceeded five or six cars. I don't know where the NP would have operated similar power except on some of there eastern trains that I am not familiar with.

The GN on the other hand operated A-A E-7 units on the Empire Builder initially later changed to A-B-A sets of F units. The Western Star operated with A-B-A sets of F units and the Internationals operated one set with an E-7 the other with an A-B sets of F units. The GN Seattle - Portland pool train operated with A-B sets of F units and the Cascadian operated with one set of A-B F units and one pair of GP7s on the other. GN Interbay was equipped to do all but the heaviest of mechanical work on the diesels.  

Al - in - Stockton