Mike - congrats on the new airbrush! You can pick up a regulator at Lowes or somewhere like that for around $20 and it should do you fine, get a water trap as well. (Moisture filter I think they are called sometimes) It's not mandatory, but unless you live int he desert running the risk of getting a big glob of water on fresh paint isn't worth it to me!
Floquil does make model railroad paints, specifically for each real life counterpart (Santa Fe etc..) They also make a lot of colors for weathering the cars realistically as well. I believe both www.greatmodels.com and www.squadron.com carry them.
The best tip I can give you is that no matter what type of paint you are using, soaking the tip in the appropriate thinner for that type of paint overnight after painting is something that will keep you spraying fairly consistently. There are many Aztek users here so if you have any detailed questions about the brush itself or its cleaning be sure and come back and ask. Good luck and again congrats on a new airbrush!!