We have a caving club that meets at the shop on the 4th Wednesday of every month. At the last meeting, someone brought in a mine car that need a new axle. So we built one. The materials were 1" solid 1018 steel bar, with angle iron welded over half of it to form a square section. A bunch of mounting holes and such then got drilled on our 1950s Bridgeport mill.
Here is Chris machining the ends and fitting the wheels:
The finished product with wheels attached: