Discussion relating to the PRR, up to 1968. Visit the PRR Technical & Historical Society for more information.
  by Allen Hazen
Ongoing discussion of duplex designs on the Steam Power forum (cf the "Thoughts on the PRR Q-2" string) got me thinking...

Various radical changes to the T-1 design have been discussed. Counterfactual scenarios are easiest to reason about, however, when they involve only a small change from actual history. So here is a proposal: one basic change, not involving any technological concepts that should have been unfamiliar to steam locomotive designers of the period, that might have made the T-1 a more useful locomotive.

The T-1, as actually designed, had 80-inch drivers. Why? Well, the PRR seems to have decided, back in the late 19th Century (!), that this was the "right" driver size for express passenger engines... and then stuck with it. Could smaller drivers have been used? Well, high speeds and small drivers lead to high r.p.m. Is that a problem?
---maybe high r.p.m. makes adequate lubrication harder. But with roller-bearing journals?
---mainly, I think, it makes balancing harder, leading to stress on the locomotive frame. But by the 1940s, with the use of cast-steel bed-plates, this was much less of an issue.
---And, of course, the whole point of the Duplex drive was to reduce the mass of pistons and rods: surely this would have made the balancing issue LESS pressing-- I would suspect that, for any desired speed, the balance and dynamic augment problems would have been lesser for a Duplex than for a conventional layout, allowing for smaller drivers.

Would smaller drivers have allowed the desired speeds? Recall that the Norfolk & Western J (a non-Duplex 4-8-4) managed 110 m.p.h. with 70-inch drivers. (And that British Rail Class 9F -- a 60-inch drivered 2-10-0 -- was run up to 90 m.p.h. on passenger trains!)

Now, the large drivers gave the T-1 a very long rigid wheelbase: 25"4'. This, all by itself, was a problem for the Pennsylvania Railroad: any PRR power, to be useful on long-distance passenger trains, had to run through Pittsburgh, and the West end of the Pittsburgh station has a very sharp curve leading onto the Allegheny River bridge -- a curve that was problematic for the T-1.

So, what if the T1 had been designed with, say, 70" drivers? I think this would have permitted a 33-inch reduction in driving wheelbase (30 inches just because of the smaller wheels, and 3 inches from the use of shorter-stroke cylinders). This would still have left a long wheelbase-- 16 inches longer than some 80"-drivered 4-8-4 on other railroads-- but might have made operation through Pittsburgh (not to mention a certain famous curve west of Altoona) easier. Without, as far as I can see, any real drawbacks.

  by Allen Hazen
I forgot to mention one more major reason for not liking high driver r.p.m. Getting steam into and out of the cylinders is a problem at high speeds: steam locomotive horsepower falls off at very high speeds, I think largely because the valves can't handle the job. But here again, the T-1 shouldn't have been as affected by this as conventional designs: poppet valves can handle the steam at higher speeds than piston valves. (The... we don't talk about that other railroad on the PRR forum, but you know who I mean... Niagara 4-8-4 was built in both piston-valve (6000) and poppet-valve (5500) versions: the poppet valve engine maintained high power to a higher speed than the piston-valve.) So, here again, the T-1 shouldn't have needed drivers as large as more conventional types for operation at passenger train speeds.

(Baldwin, before PRR management agreed to buy the T-1 prototypes, had been thinking of building a Duplex demonstrator: like a T-1 in basic specs, but with piston valves. Larger drivers might have been more valuable her... though, the smaller cylinders of a Duplex would have helped, making it easy to use piston valves that were larger, in proportion to the cylinders, than usual: sort of like the K4A approach to improving performance of the K4. PRR had experimented with poppet valves on a K4, and so knew the best way of achieving high power at high speed!)
  by jaygee
Interesting idea. I'm thinking that you might have seen two possibilities here; One with 77" drivers and another with 72"
drivers. Simply because they were already in stock, when the production locomotives were built. I doubt you would have seen a lot of difference in performance, assuming the PRR Mechanical Dept. in Altoona had set the machines up to maintain the same ratios of tractive effort and factor of adhesion. A lower top speed would likely result, but that would have been largely theoretical, given PRR never really made any attempt to capitalize on the speed potential of the T1. I began thinking about this largely by accident, back in the late '70s, when Alco KMT released a T1 modernized in HO scale with 76" drivers. A freight T1??? Dual sevice?? Anyway others have commented that such a machine would not be a necessity on the PRR, as the M1b class was initiated after WW2 and did everything the low driver T1 would have done.....except leave everyone else in the dust
at 125 MPH !
  by Allen Hazen
Thanks for comments!
I chose 70" when I wrote because (i) I wanted to minimize rigid wheelbase: 77" wouldn't have made as dramatic a difference, and (ii) PRR did have modern locomotives with that driver size: the J-1. (I don't know if there would have been any parts commonality: given the very different machinery weights, and so balance-weight requirements, J-1 and "T-1-70" driver centers would not have been interchangeable, but perhaps they could have used the same tires.

As you point out, the PRR never made any real use of the high-speed potential of the T-1. Given the comparatively light weight of rods and pistons, and so the possibility of very good balancing, I would think that a 70" drivered T-1 would have no difficulty with 100 mph, which was (wasn't it?) above the timetable speed limit for PRR passenger trains (except maybe in the electrified district? I know many GG-1 were geared for 100 mph top speed). (Come to think of it, the GG-1 is maybe relevant here: PRR,in designing the T-1, wanted a unit that could do in non-electrified territory what the GG-1 did under wires, so there was no real need for a higher top speed than that of the GG-1.)

As to comparison with the M1b... Interesting thought! At agues, the M1b (with piston valves) might have had its peak power output at ... ??? ... ??? maybe 60mph (that's a total guess): the T1 would have been more powerful at maximum passenger speeds on the Ft. Wayne Division.
  by jaygee
I doubt the Penn would have seriously considered 70" drivers for the T1 as they thought this size was too low on the N&W J Class...which was tested at speed quite successfully. And you are correct about the GG1 being the paradigm of passenger power performance. Did the T1 really need to achieve 140MPH? No! But they could and they did, so those in the know would wait until they got the right train (like mail #44) heading out across the Indiana flatlands with a T1 in good shape....and proceeded to tear up the midnight countryside! Naturally the Penn management was less than enthusiastic about this behavior, as was Lima owned Franklin Railway Supply, who's poppet valves turned to ashes under these conditions. The poppets were eventually redesigned to survive in the rarified atmosphere Penn sometimes put them in, but working on them was always a pain, except for #5500, with her Rotary "B" poppet gear. The physical plant itself was in no shape to handle these speeds, regardless of the track itself. T1 powered passenger consists at these speeds would hit grade crossings before they could even begin to activate!
Some of the cars in the trains, such as the lightweight observation cars PRR built simply wouldn't behave at speeds over 100MPH ! OTOH., What IF the Penn had decided to capitalize on this trait?? Short, high speed trains across Ohio, and Indiana could have been achieved and marketed in the late '40s and early '50s. A consist like the Budd "Keystone" was nifty with a GG1, what would it have been like with a T1???? In a word...Outrageous !
  by Allen Hazen
"I doubt the Penn would have seriously considered 70" drivers for the T1 as they thought this size was too low on the N&W J Class...which was tested at speed quite successfully."

Well... I think we're seeing the problem: the decision-makers didn't appreciate the changes the newer technology they were about to invest in had made: they still thought high drivers were necessary for high speeds, despite the greater rigidity of modern (cast) frames and the reduction of balance-problems made possible by the lighter weights of Duplex machinery. (PRR's people weren't alone in this: over on the New York Central, the Niagara's were equipped with 79" drivers, thugh one or more were tested with 76": the thinking seemed to be "79 inch drivers for passenger speeds:thats what the K3 and the J1 had, so it has to be right for the S class!" I think it's clear that the Niagaras-- particularly the poppet-valved S2-- would have been just as good, perhaps a bit better, with the smaller drivers.

Basically I think that no serious thought had been put into new steam designs during the depression and WW II, and when the PRR and the NYCR bought their final steam passenger units their thinking was constrained by obsolete assumptions.

Sorry, that was a rant.

Interesting that the PRR had at least thought about the N&W J design (I guess, when you think about it, it's obvious that they WOULD, given the close corporate relations between the PRR and N&W)... and refused to believe that it was as good as it was!