I came across a rusting technique a few years ago using a Rust Antiquing Set from Sophisticated Finishes. This is a two-part system used by interior designers and architects to age items such as iron, bronze and brass, however, it can be used on plastic with very realistic results. These sets can be found at arts and crafts stores or online at patina.com.
Like all new techniques, I would advise practicing on a cheap spare wagon or on some scrap plastic card until your comfortable with it. The material does create a realistic texture, which in 4mm, OO, HO and smaller can appear a bit over scale.
To use the Rust Antiquing Set, paint on the metallic surfacer, let it dry for 24 hours and then apply the rusting solution, let it dry and your done. If after one application of the rusting solution you don't feel its rusty enough just paint on some more.