For you, I'd recommend two possibilities. The Iwata revolution CR or the BCR. You can see them here:
https://www.iwata-airbrush.com/revolution-4500-cr.html
and here:
https://www.iwata-airbrush.com/revolution-2000-bcr.html
To give you some help choosing between gravity feed and siphon feed, the 1/3 oz. cup on the Revolution CR will hold enough paint to do two coats on an HO 50 ft. double door boxcar, so think about what you'll be doing and pick the style that best fits your needs.
They are in the hundred buck range, which means you may have to put off your next locomotive purchase for a bit

, but they have the following going for them:
You'll only buy the one. I still have my very first Iwata, and all I've ever done with it was needle nozzle replacement. The initial nozzle set lasted 5 years and I replaced them because I dropped it. Yeah that happens to the best of us! You can probably do considerably better than the hundred buck range with the Hobby Lobby 40% off coupon or similar offers. I like this brand because the needles and nozzles are hardened stainless steel, not brass lime most others, so they last a long time. Machining is superior, so I didn't have to use beeswax to plug air leaks like I did with a Badger Patriot I bought. They use o rings in their air fittings, so everything just has to be finger tight. The Revolution has a .5 mm nozzle size which is important for using acrylic paints.
You'll need a hose too, and that's typically another $15-20.00. There are several types. I use the braided one because I stand and it's tough. There are also urethane hoses that are almost as tough and lighter. it depends on what your work area is going to look like.
No matter what brand you get, thinning paint will be the most important thing to learn. Remember the milk rule. Thin your paint until it's the consistency of milk. It should leave a film on the bottle or mixing cup just like what's left when you finish a glass of milk. Feel free to ask any other questions you may have. I've been doing this for a long time, and made all the mistakes!

Do you by chance belong to Trainmasters TV? I did a series of airbrushing shows for them you can watch. Those were a lot of fun!