This video explains how to add a weathered effect to rolling stock.
Humbrol products used in this video;
Humbrol Weathering Powder
Humbrol Matt Cote
Humbrol Enamel Thinners
Humbrol Clear
Humbrol Acrylic Spray Matt Varnish No.49 - AD6049
Please take extra care when applying Weathering Powder and any Varnish to your locomotive. Hornby Hobbies Limited takes no responsibility for damage caused when weathering your model. We strongly suggest you practice the techniques using a test piece first.
http://www.humbrol.com/humbrol-weathering-powders/