It was quite simple and involved a 2-part product called Instant Rust that was marketed by a company called Modern Options (www.ModernOptions.com).
It is now sold under the name of Sophisticated Finishes Rust Antiqueing Set (www.Tricoat.com).
Just paint on the metallic surfacer, let it dry for 24 hours and then apply rusting solution and let it dry and your done. If after 1 application of the rusting solution you don't feel its rusty enough just paint on some more rusting solution.
Now like all techniques I would advise practicing on a cheap spare wagon or on some scrap plastic card until your comfortable with it. The only other caveat I can think of is that it can appear overscale in 4mm/OO/HO if you're not careful.
Here are a couple of pics of a pair of HO gondolas I did 8 years ago....be advised that I'm no photographer so the pics don't do the gondolas justice...they really do look much better in real life