Related to my new factory chain post, i'm wondering how to make vehicles etc for simutrans..
It's loong to explain, but you can start drawing the eight views needed (S, SE,E,NE,N,NW,W and SW) then we can talk about the dat file. Have a look at the SVN (http://simutrans.svn.sourceforge.net/viewvc/simutrans/pak64/vehicle/) or graphics.simutrans.com (http://graphics.simutrans.com) for more examples.
as vilvoh says, you need to end up with a png and a dat for the vehicle as in the examples on SVN. Two main methods to do the images:
1. 3D model rendered 8 ways (e.g. blender) - I do my stuff this way because it doesn't require any drawing skill and allows detail quite easily.
2. Pixel paint (i.e. draw in a paint program pixel by pixel).
The one drawback of rendering method is that you can't get the "player colors" on the model easily (am I right, or is there a trick for that ?). That applies only to vehicles and stations of course - industries and city buildings don't need "player colors".
@dantedarkstar - player colours are not easy in rendering - you will notice pakBritain doesn't have them, there is a reason for that ;)
Raven used to render in black+blue and then reduce palette to Simutrans specials+grayscale. It worked great.