wjstix posted:Back to the OP, regarding the lead truck, it has to do with "hunting" as Rich pointed out. A big factor in that is speed. An 0-6-0 or 0-8-0 moving slowly around a yard isn't very likely to derail at a switch. An engine going faster is going to be more likely to have a problem, so adding 2 leading wheels is going to help steer the engine through switches and around curves. The fastest steam engines were passenger engines, and they used 4-wheel lead trucks to ensure they safely negotiated crossovers and curves at high speeds.
The other very important function that a lead or engine truck performs, besides guiding into curves & turnouts is, flange wear on the front drive wheel/wheels is drastically reduced. Even locomotives equipped with lateral motion devices would still experience excessive flange wear without any sort of lead truck.
In an engine with no trailing truck, the entire firebox has to fit between the drivers. By adding a trailing truck, the firebox can now be the full width of the engine, giving you more combustion and more power.
