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[rjemery]

Given a steam locomotive, say a "Northern" 4-8-4.  Is all the weight of the locomotive on the drive wheels?  Are the pilot wheels or the trailing truck load bearing as well?

Also, why would there be a need for more than 2 pilot wheels if the only job of the front wheels is to help steer the locomotive around a curve?  Ditto for the trailing truck.

[michaelstevens]

As (the design of) steam locomotives evolved, the designers came up with a myriad of combinations of boiler sizes, fire grate areas, cylinder sizes and quantities, wheel arrangements etc. in their pursuit of the perfect engine for a given task.

They started out with an 0-2-2 (the Rocket) and an 0-4-0 (Locomotion) but quickly discovered that a leading truck enabled a longer (and heavier) boiler with outside cylinders, while improving the locomotive's ability to negotiate curves at speed  --  e.g. the 4-4-0 American.

The trailing truck was found to be useful for helping to carry the load of a larger fire box  --  e.g. the 2-6-2 Prairie.

4-8-4 Northerns are generally considered to represent the ultimate in steam motive power for express passenger or fast freight trains and

1.   No all the weight of the locomotive is not on the driving wheels

2.   Yes the leading and trailing trucks both do bear part of the load

3.   The number of axles in each truck are determined primarily by the amount of load required to be borne (railroad track and structures all are designed to support finite axle loading limits)  --  many locomotive designs included 3 axle trucks (e.g. Pennsys' 6-8-6 S2).

There are numerous books out there on this subject  --  one I have is "Steam Locomotives" by Luciano Greggio (Hamlyn Publishing/Crescent Books 1985)