I suspect that with a restored 10-coupled locomotive, there might be some difficulty in finding lines over which it could be allowed to operate, given that as a group, this type was generally viewed as being hard on the track and roadbed, and on underline bridges.
A lot of that expressed concern was in respect of vertical track loading resulting from hammerblow and piston thrust. In this regard, a while back I found a most interesting article in the R&LHS Newsletter Volume 24, No. 3, Summer 2004, available on-line at: http://rlhs.org/Publications/Quarterly/PDF/nl24-3.pdf
" onclick="window.open(this.href);return false;. This was “Counterbalancing 10-Coupled Power”, by Don Leach. From this one could infer that for any restored 10-coupled locomotive, it would be prudent to use the light-weight rods, disc drivers and the counterbalancing techniques that prevailed at the end of the steam era, even if the locomotive concerned was not actually built or rebuilt with these features.
The article also answered a couple of long-standing questions, which were: (1) why did the UP’s allowed axle loadings in the steam era lag behind those of many other of the larger Class I roads; and (2) why was the UP 2-10-2 about half-a-size smaller than say the SP 2-10-2, which was proximate in size to the USRA Heavy 2-10-2. (Both the UP and SP examples look as if they could have been Harriman-heritage designs.) To the first question, UP evidently restricted its axle loadings because of its desire for relatively high operating speeds; for a given axle loading, track damage increases with speed. Thus the limits were, I think, successively 59 000 lb for the 2-10-2, 4-10-2 and 4-12-2 designs, 65 000 lb for the first Challengers and 67 500 lb for the FEFs, Big Boys and “big” Challengers. The UP 2-10-2 was designed both for the 59 000 lb limit, and for higher operating speeds than was customary for this wheel arrangement. The 30-inch piston stroke, rather than the common 32-inch number, was one factor that helped along this vector.
The larger 2-10-4s tended to have very high axle loadings, perhaps towards 10% higher than the roads that ran them allowed for their heaviest 8-coupled locomotives. Insofar as these roads were using 2-10-4s for tasks for which elsewhere the 4-6-6-4 might have been chosen, one may see the need for the highest possible adhesion weight. The above relativity shows up when one compares say the C&O T-1 with the Van Sweringen 2-8-4s to which it was related. (For example, the NKP 2-8-4 was said to be 70% of a T-1, not the 80% that the relative driver count would indicate.) Still, I have wondered whether the “excess” axle loading of some 2-10-4s was the result of an effort to reduce the dynamic augment-to axle loading ratio. If the former was essentially fixed as the result of the dimensions required to transmit the desired power, then axle loading was the only available variable which could be adjusted accordingly, that is upwards.
Less seems to have been written about lateral railhead forces, but it is difficult to imagine that most 10-coupled locomotives would have been anything but unedifying in this regard. Clearly the long rigid wheelbase was a disadvantage. But since virtually all had two-wheel engine trucks, then probably quite proportion of the nosing and curving forces were taken by the leading and second sets of drivers. In hindsight, it is perhaps surprising that in later years, as higher operating speeds were required, that there was not some move to four-wheel engine trucks, and possibly also the Alco-Blunt “lever principle” in which in a set of n driving axles, the first (n-1) all had lateral motion devices with appropriately graduated compliances.
Of course, the SP and UP had the 4-10-2 wheel arrangement for their respective three-cylinder 10-coupled designs (which bore about the same relationship to each other as did their 2-10-2s), but that I think was necessitated primarily by the need to carry the extra weight of their three-cylinder front ends. Still, that experience seems to have convinced UP that four-wheel engine trucks were highly desirable for freight locomotives. (Although one could counter-argue – maybe not all that strongly - along the happenstance vector that as on the 4-10-2, it was a necessity for the 4-12-2, from which it was carried over to the first Challengers which were in some respects a “bent” version of the 4-12-2 – with the same grate area - but with a four-wheel trailing truck to carry the extra weight.) Anyway, as the 4-10-2 was a reality, it is not so difficult to imagine a 4-10-4.
As to the “lever principle”, I don’t think that it was all that widely employed, at least in full. UP, on the FEF-2 & 3 (not sure about the FEF-1), Big Boys and big Challengers, and D&H on the K-62 are the examples that readily come to mind, but not any 10-coupled designs. Perhaps the roads that operated heavy 10-coupled locomotives had in early days adjusted to probably higher intensity of track maintenance required where they operated, and simply accepted it as a “fact of life”, or as their chosen trade-off against the probably higher locomotive maintenance intensity required with articulateds.
In view of the tracking and curving difficulties, perhaps surprising is that 10-coupled locomotives were used on some Cape and metre gauge roads. Of note is that metre gauge 2-10-4s and 4-10-2s (three-cylinder) were operated in Brasil.
Returning to the main theme, would a restored AT&SF 2-10-4 be allowed to operate over the route it was originally designed for, Belen to Clovis, NM?