Regarding the difference between:
2878c01 Train Wheel 2, Complete Assembly
2878c02 Train Wheel 2 for RC Train, Complete Assembly

c01 has a metal axle that goes clear through the center of the wheels.
c02 has a metal axle that does not (and the wheels are easily removed from the axle when they're removed from the housing).

This Item is similar in mold type to the following Item(s):

• Part 2878c01 (Inv) Train Wheel RC Train, Holder with 2 Black Train Wheel and Chrome Silver Train Wheel, Metal Axle with Slots (2878 / 2879 / bb837)