A flat finish on wheelsets and trucks can make a big difference to the realism of rolling stock. Recently I found an easy way that makes a durable finish.
After giving wheels a cleaning with isopropyl alcohol I use Woodland Scenics Weathered Tie track painter pen to paint the wheelsets. I like to give them two coats.
I like to paint truck side frames with Rustoleum Camoflage Brown. It sticks to slippery plastic and makes a durable flat finish.