Mike,
If this is your first attempt at re-painting a train i would advise against an airbrush no matter how inexpensive. Why? There is a learning curve involved, plus the airbrush is only part of the cost; even if you have a compressor you will still need a water separator and the appropriate fittings/hoses to attache your airbrush.
Inasmuch as you're not really planning any more painting in the near future (if ever), and the fact that the paint scheme is very simple (all black with white decals), i'd strongly recommend a rattle can of Scalecoat II, practice on a piece of primed plastic, moving the can in a smooth rhythm, at a distance until you can lay down a nice thin coat. Once you get the hang of it, paint your engine.
If a rattle can doesn't appeal to you try buying a Preval sprayer for less than $10, a small jar of Scalecoat II, a can of Scalecoat II thinner, practice a bit and go at it that way. Total for everything less than $25 not counting primer.
However the least expensive approach would be one can of Testor's light gray primer and a rattle can of Scalecoat II paint - less than $20.
Remember whatever approach you use, there will be a learning curve involved, so why not go with the least expensive instead of jumping in when you don't need to yet - unless you're made of money 
. Some folks here seem to think nothing of throwing money at their hobby; i like solving problems as inexpensively as possible, and my RR will stand up quite well compared to most and has been featured by my local NMRA Division (a predominately N & HO group of which i'm not even a member) 3 times on their layout tours. As one of my Scottish friends always says, "Money is a lot easier to save than it is to earn".
jackson
https://www.youtube.com/watch?v=gJmTzIFKvys
jackson
