• ALCO FAs vs PAs

  • Discussion of products from the American Locomotive Company. A web site with current Alco 251 information can be found here: Fairbanks-Morse/Alco 251.
Discussion of products from the American Locomotive Company. A web site with current Alco 251 information can be found here: Fairbanks-Morse/Alco 251.

Moderator: Alcoman

  by Snowmojoe
I am curious about the differences between ALCO FA/FB units and PA/PB units. As I understand it, and correct me if I'm wrong, that FAs were freight locos, and PAs were passenger locos. Is there any particular reason, be it mechanical, cosmetic or otherwise, that this was so?

I believe PAs and FAs were compatible as far as MU link ups go, so was it common for these units to operate in mixed lots, or was that just never done?
  by Desertdweller
FA/FB = freight
PA/PB = passenger

These aren't two versions of the same thing. They are two very different models.

The FA/FB is much shorter, uses 2 axle trucks, four motors, and a 1500/1600 hp V12 engine.

The PA/PB uses 3 axle trucks with two motors each and an idler axle, and a 2,000 hp V16 engine.

Like almost any Diesel loco, they will MU with each other and just about everyone else's loco. They were mixed with other locomotives, usually with other passenger or freight units, but can be mixed freely.

FA/FB's were ALCO's competitors to the EMD F-units.
PA/PB's were the ALCO competitors to the EMD E-units.

Both FA/FB units and F units were available equipped for passenger service as an option.

  by scottychaos
FA's and PA's *could* MU and operate together, in theory,
but in practice this very seldom happened..
For the majority of their careers, PA's were primarily passenger units, and hauled passenger trains.
and for the majority of their careers, FA's were primarily freight units, and hauled freight trains.

As DD said, there were "passenger variants" of the FA, called FPA units, with steam generators and different gearing,
but "straight FA's" were meant for freight.

Later in their careers, the PA's of several railroads were used in freight service, so it is possible to find photos of PA's hauling freight,
but that was a rare exception.

As a general rule, 99.9% of the time, you would not find find PA's and FA's mixed together on the same train.

  by Allen Hazen
Diesel locomotives are geared for specific speeds. (The traction motors-- the electric motors that actually turn the wheels-- are geared to the axles. With freight locomotives, typically the motor rotor rotates about four times as fast as the wheels.) Different locomotives are geared for different speeds: passenger locomotives typically have a lower gear ratio, so the same speed of the electric motors corresponds to a higher rotation rate for the wheels, and so a higher locomotive (& train) speed, than freight units.

Putting together several locomotive units in multiple works best when the different units all have about the same gear ratios (& so about the same top speeds). Most PA units had gearing for higher speeds than most FA units (though a couple of railroads-- U.P. and Pennsylvania, I think-- regeared their PA units when they started using them mainly on freight), so good operating practice would not havecoupled them together on a regular basis.

Note that the PA had a 16-cylinder engine, and so about a third higher horsepower, than the FA with a 12-cylnder engine. In both cases only four axles were powered. (The middle axle on a PA's 3-axle truck was just to spread the weight: it didn't have a motor geared to it.) So the PA had more power per (powered) wheel than the FA: good for high-speed passenger trains, bad for lower-speed freight: at lower speeds a PA using full power would spin its wheels uselessly.
  by Snowmojoe
OK... I figured there had to be a mechanical/technical reason that these distinctions were made. So the gearing on the locos is kinda like the gearing on a 4x4 truck; you can't have two axles with different gear ratios, b/c then you'd have wheels turning at all different speeds?

It seemed just from looks that the PA units made better sense for freight, b/c you figure larger size = more power; freights are heavier than passenger trains. But the speed thing never really occurred to me, I suppose.

So, if you re-gear a PA for freight service, how does that solve the wheel slippage thing? Isn't wheel slippage a product/derivative of axle load? And the A-1-A trucks on a PA would mean lower axle load for the PA...
  by Allen Hazen
Modern (diesel, North American) freight locomotives have over 700 hp per powered axle (4400/6). They can manage with this-- can use their power to pull trains instead of having to reduce power to avoid slipping-- because (a) they have very sophisticated, microprocessor-based, wheelslip control systems that detect the beginning of a slip and correct for it, and (b) they have lots and lots of weight on the axles (CSX's "AH" locomotives weigh 436,000 pounds, so over 70,000 lbs per axle). The PA and FA didn't have either of these advantages: control electronics in the late 1940s weren't that sensitive, and these locomotives weighed between 50,000 and 60,000 pounds per axle.

So they just couldn't use very much power. The PA, designed for passenger-train speeds, had 500 hp per powered axle, but even that was too much at lower, freight-train, speeds: so the FA-1 had only 375 hp per powered axle. (Note that at the speed of a fast freight train on fairly level track, the PA's greater power could be used. But slower freight trains (think coal and ore), particularly going up hill, spend a lot of time running at much less than 20 mph, and freight locomotives were designed for this.

You are right that changing the gear ratio doesn't solve the wheel-slip problem. Wheel-slip depends on the power delivered (which determines how much force has to be exerted), the speed, and the weight on the axle. What the higher gear ratio does is increase the motor r.p.m. at a given wheel r.p.m. Put a lot of power-- so a lot of electric current-- through a motor and it heats up. For a given amount of power, this effect is worse at lower motor r.p.m. (Amps is what heats things up: the way traction motors work, at higher r.p.m. the voltage is higher, but the amperage is lower.) Running a passenger locomotive at full power at low speeds risks heat damage to the traction motors: PRR and UP re-geared their PA locomotives when they re-assigned them to freight so that the motors would "think" they were operating at passenger train speeds.

F.W.I.W. Fairbanks-Morse, in the mid 1940s, designed a locomotive very similar in specifications to the PA (A1A trucks, 2000 hp diesel engine): the "Erie-built," so-called because it was assembled by General Electric at the Erie, PA, plant. (The similarity to the PA is maybe no coincidence: Alco was in a partnership with GE in the locomotive business at the time, and the same GE engineers who worked on the F-M locomotive were involved in designing the PA.) They marketed it as a dual-service locomotive, suitable for both passenger and freight trains. But the ones sold for use on freight trains were ballastedto be heavier than the ones sold for passenger use.
  by Desertdweller

The effect of mu-ing units with different gear ratios would not be as bad as a 4x4 truck with different gear ratios, because the electric transmission in he locomotives is not a direct mechanical drive. It would work, but not as well as if the ratios were matched.

When I worked at a Ford dealership, we get a brand new 4X4 that would work fine in two-wheel drive, but was undrivable with 4x4 engaged. Turned out the front axle gear ratio was not compatible with the rear ratio.

Actually, ALCO did make a locomotive like you described. It was a PA-style locomotive, but with C-C trucks instead of A-1-A trucks. This gave it 6 motors to soak up the horsepower. Unfortunately for us, this model was export-only. I don't know the model number, or if any still exist. If so, one of these would make a nice addition to a rail museum or tourist line.

The Fairbanks-Morse Erie Builts had much in common with the PA. So much so that the trucks under Doyle McCormick's restored PA came from an Erie-Built.

  by scottychaos
Desertdweller wrote: Actually, ALCO did make a locomotive like you described. It was a PA-style locomotive, but with C-C trucks instead of A-1-A trucks. This gave it 6 motors to soak up the horsepower. Unfortunately for us, this model was export-only. I don't know the model number, or if any still exist. If so, one of these would make a nice addition to a rail museum or tourist line.

Alco DL500 and variants, made in the early to mid 1950's

http://www.railpictures.net/viewphoto.php?id=212775" onclick="window.open(this.href);return false;

http://www.railpictures.net/viewphoto.php?id=134436" onclick="window.open(this.href);return false;

There are still some around in Europe.
  by mandealco
There are, or were until quite recently, DL-500 units running in Australia, the 44-class of NSWGR origin.
  by Desertdweller
Thanks Scotty, Steve!

Wouldn't it be nice to have some of those over here?

They appear to be FA's with elongated car bodies.

DL500. I'll remember that!

  by Allen Hazen
The Dl-500 (a.k.a. the "World Locomotive") was built for the export market: most non-North American railroads had much lower axle-loadings than U.S. railroads, so the Dl-500 had 3-axle trucks to spread the weight. Internally, however, it was more like an FA than a PA-- in particular, it had a 12-cylinder engine (it was initially designed around the 12-244 used in the FA, but almost all the Dl-500 actually built had the later 12-251, as used in the RS-11... or the MLW FPA-4), not the 16-cylinder engine used in the PA. I believe that, at the time the Dl-500 was introduced, Alco was building an order of modified (3-axle trucks to spread the weight) FA units for Pakistan... and switched to the Dl-500 design for the end of the order.
We are now used to very powerful freight locomotives, but in the 1940s the "standard" was to use comparatively low-powered units (EMD's FT was only 1350 hp!), coupling as many as needed in m.u. to pull long freights. Alco didn't try to use a 16-engine on a freight locomotive until the Dl-600 of 1954, by which time it seemed better to use a hood-type construction rather than a PA- or FA-like carbody. ... The Dl-600 had 3-axle trucks, all axles motored, so in terms of puling ability it was roughly equivalent to one-and-a-half FA units. There have GOT to be cost advantages to using a smaller number of higher-horsepower units: it's a bit of a mystery that Alco didn't think of a six-axle freight locomotive, with a 16-cylinder engine, earlier. EMD, with its low-output, non-turbocharged, engines would have had a hard time coming up with an economical competitor!