This is an excellent question asked down in the painting, decaling and detailing forum below. Do you have an airbrush? What type of paints do you want to use (acrylic, enamel, etc...)? If you are going to use acrylics Floquil and Polyscale are not long manufactured, but you might be able to find some using Google search. Tru Color makes road specific paints (PRR, NYC, etc...), but are a bit more difficult to use and clean.
What type of decals do you have, or want to buy? Water or dry transfer? I prefer water transfer because they can be more precisely positioned after application. Dry transfer are in place when applied. If you are going to use water transfer you will need a bottle of something like Micro Set and Micro Sol. Micro Set aids in getting them in place and adherence. Micro Sol softens the decal so it will conform better to the surface. Micro Scale is a popular brand of decals. They carry O, HO and N scale decals and also the Micro Set and Micro Sol. I believe the minimum order is $25 if you buy decals directly from them. You can also find them out on eBay.
Testors has a line of paints specifically made to match railroad colors. Anymore, I prefer acrylics. Probably want to wash your plastic with soapy water and let dry thoroughly before starting. Plan out what you're going to do. Hopefully you're using an airbrush because the quality difference is well worth it.
Get some clear coat and a bottle of decal set. I spray the clear coat over flat paint. It dries, you can't see it. But its still flat and provides a nicer smoother surface to apply the decals, which helps eliminate air bubbles. Then I throw the decal in water for 10 seconds, then let it sit on a paper towel for a minute. The decal will slide off the paper and take some of it's glue with it. While it was sitting, I brush on some decal set to the area that's getting the decal. So the surface is wet.
Then with a tweezers, (be sure to have a good selection of small tweezers. Sometimes I have a pair in each hand) I drop the decal on and slide it around. Then I'll use a small piece of paper towel to press it in place or push the liquid out with the brush. Then I'll brush the decal setting solution over top of the decal and around the edges so it lays down real nice.
I just used this method to replace the logo on a tender and it worked great. As the decal dries, it will get wrinkly. I was really disappointed when I saw this. But it kept drying and flattened right out. It also laid down tight and flat over lines and rivets. I credit the decal setting solution. I don't think it would've been as nice without it. I've applied a lot of decals in my life but just started using this stuff.
Micro-scale is definitely the place to get what you need.