How to use the various wagons

General railway mining

Just like other mining equipment if the ore requires fluid then you'll need a suitable fluid wagon to provide that fluid as well eg mining uranium requires sulfuric acid.
Ore mining wagons can gather ores directly from the wagon on the tracks being near a ore patch.
Fluid mining wagons can gather fluids such as crude oil and water.

Fusion powered wagons

Fusion-powered mining is self-powered. Just assemble and use. Fusion mining still requires associated mining fluids.

Steam powered wagons

Attach the mining or fluid wagon to a train that has a fluid wagon containing steam. If the train runs out of steam, the steam-mining wagons stop mining.