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So a train as the background, then Grafitti type art all on a canvas? Now should i go for coloured pencils, or watercolour and marker pens, acrylic paint? How do you reckon I should go about it?

Vote - Acrylic.
You can give it real depth and grunge on an old rustbucket vintage steam train engine that got a modern graffiti treatment.

I definitely do like that idea, once I finish the current bit I'm working on I'll get onto it, especially like the idea of having a steam train with a modern touch up!

Yeahhh I like penderis' idea !!! Cant wait to see what ya come up with !