I do the same as sherm , except one thing , I use a product called" sophisticated finishes" iron surfacer and rust in a bottle , theres nothing like using the real thing to make a simulation , this stuff has iron powder in it , its slightly grainy when painted on , you let it dry then apply the ruster and there you have it , rusty tracks ! I also hit them with a thinned out humbrol dark or light earth airbrushed on to simulate just that