Perhaps what is not obvious is that a steam locomotive that has at least one weight-bearing truck, whether it is a trailing truck under the firebox and cab, or whether it has one under the front pilot to aid in tracking at speed/carry weight of the cylinders, smoke box, and pilot, the total weight of the locomotive doesn’t bear down on the driving wheels…as is the case with diesels. So, the more weight you can pile on an engine that must have a weight-sharing truck or two, the more weight you get on the drivers for traction. By placing the water reserve on the locomotive, whether in saddle configuration draped over the boiler, or as side saddlebags on either side of the boiler, the extra traction right over the drivers on these tank engines really helps them to work on uneven and hilly tracks.
Shay and other geared locomotives have all of their wheels driven via rods and gearing, so all the weight has a bearing over the driven wheels…much like most diesels. They may be slow, but they can really handle crappy temporary track, and pull large tonnages along them.
-Crandell
