Also, there are books and magazine articles on the subject - especially in the
non-3-rail-O-world, but painting is painting.
After looking around on this Forum as Lee suggested, and maybe scanning the
Net for articles/publications (try searching "painting model locomotives" and
check your hits), try something simple and cheap - like a boxcar. Get some
decals from Microscale or from daBay. Spray cans can be fine - you don't need an
airbrush. Learn to use Solvaset and Dullcote and the like.
But do that "cheap" boxcar and caboose first; like driving a car, explanations are
great, but you need to drive around the block to get the feel. Wait awhile for that long trip.
I have no publication title to recommend, but I personally prefer a single reference
source on a subject to various and varied opinions, especially if you are a novice.