Wheel Diameters

A previous post of mine got me thinking of this question - Are all axles on railcars the same size? Is there a standard size for freight and passenger equipment?

no, most MU cars had 33" or 36 " wheels heritage cars are 40" most of time, Locomotives were 40" but with higher horse power they have been going for 42" wheels

Why do MU cars come with smaller diameter wheels?

not enough-room for truck and maintain floor height for high platforms. the main reason locomotive wheels are bigger is to accommodate bigger traction motors, and floor height is not a problem.

Larger diameter is also better for weight (the area of contact between wheel and rail is increased): I believe that the heaviest freight cars have slightly larger wheels than older freight cars. (And I have certainly seen suggestions that larger wheels should be used on future heavy haul cars.)
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Related question. A GE locomotive maintenance manual available at "Fallen Flags" has a note about permissible variation of wheel diameter: wheels on an axle should be the same diameter, but wheels on the different axles of a locomotive can vary by up to "25 tapes". What's a tape?

Wheel size (diameter) for cars is primarily a function of floor height. For locos it is for TM size and adhesion factors (IIRC there are 44" wheels on some ACs). Vast majority of US freight cars today are on 33" or 36" wheels. Some autoracks/intermodal flats are/were on 28" wheels due to low floor height. There are fewer sizes today than in the past for freight due to standardization. Axle size and wheel (plate) contour varies due to axle load requirement.

Allen Hazen - A 'tape' as referenced in the GE manual is a division on a Pi tape (wheel diameter is actually measured by circumference, with the measurement increments on the tape converted to corresponding diameter). I don't recall at the minute if the standard divisions for RR tapes is 1/16" or 1/10" diameter or what. Been a while.

Train Detainer-- Thanks for reply! I figured it was something like that (putting a tape around the wheel is surely easier to do than trying to measure diameter directly with any accuracy!), but had no idea what size the units were. 1/16 inch makes sense: I think diesel locomotives can tolerate over an inch of difference in diameter between wheels on different axles, and the manual said "25 tapes".

Articulated 3 and 5 unit well cars ("double stacks") often have different wheel diameters on the same car. End trucks will have standard 33" wheels. However the intermediate trucks (trucks the articulating points and which carry approx double the load of the end trucks) will have 38" wheels.

Makes for an interesting audio effect when one of these cars with slide flatted wheels passes at speed - the end and intermediate wheels sets bang away at different frequencies.