MicroMark has a great online tool catalog for hobbyist You can also request one of their printed catalogs as well. I get one about once a month or every other month.
Thinking about the things I use the most with my trains:
A good set of small screwdrivers, regular and Phillips head. Small needle-nose pliers. Small wire snippers, plus a decent-sized pair for larger gauge bus wire. Insulation strippers. A decent soldering gun or iron, depending on your preference. A decent multi-meter, but you don’t need to go overboard on that.
I’ve got a lot of carpentry tools, but I didn’t buy them specifically for doing work on the trains. I did buy a little clamp for holding boards at right-angles while I fastened them together, though. I use a good square, straight-edge and a utility knife with replaceable blades a lot.
My specialty tools are the Xuron rail nippers (used for nothing else but cutting rails) and the Kadee coupler pliers and spring pick. Of course, a Kadee coupler gauge can be counted as a tool, too. I bought a hot-wire cutter for foam. If I had it to do again, I’d get the “knife” type next time. The one that’s got a wire stretched across two prongs only has about a 6-inch reach.
I use small, cheap paintbrushes from the craft store. When they wear out, so what? I go get another one. I’ve got a putty knife and a caulking gun, too.
A Dremel rotary tool. I bought it for the trains, and I don’t know how I lived without it.
I couldn’t live without my sets of jewler’s screwdrives, pliers, and files (all are Sears Craftsman). In addition, I have several standard size Craftsman screwdrives, pliers and files. As well as utility and X-acto knives. I also recommend a Dremel with a flex shaft (I love mine).
I get my specialty modeling tools, (sprue cutters, rail nippers, pin vise and drill bits, etc) from my LHS.
I have a soldering gun (for trackwork) and an iron (for everything else). I also have both a digital and analag multi-meter. I use the analog the most (it’s easier to read). For wiring, I also have an automatic wire stripper, and a crimping tool.
I use inexpensive $2 paintbrushes and craft paint for scenery work.
You can get a good compressor with an air tank, regulator, and air trap for a fraction of the cost of a dedicated hobby compressor at Walmart, Sears, Home Depot, or any DIY store. Some arts and craft stores such as Pearls or Michaels and some DIY stores carry Xacto, Mascot, and other brands of tools useful to hobbyists and sell at a discount. Micro-Mark, although pricey, is another good source for useful and unusual tools.
As stated above, Micro Mark is a good source of the small, specialized tools you’ll need.
Several companies now offer small compressors for under $100, much cheaper than a regular air brush compressor and capable of driving small air tools. All of them I’ve seen come with a regulator, so they can be set for the different tools. Check Sears or your local home center. Watch for sales.
While you’re at Sears tool department, check the screwdriver section. They sell a nice precision screwdriver set, sizes most needed by modellers, and they’ve even come out with a cushion grip version since I got my set. Plus, they sell them as individual screwdrivers as well. Then check out the pliers, they also have a nice set of stainless steel precision pliers with grips in a nice folding soft case.
And always a good bet, a good cordless drill, at least 14.4 volt size.
Beyond that, how much more are you interested in? There’s a lot more that can be added to the list if you’re talking about building benchwork, laying track, wiring the layout, etc.
Get an OptiVisor #5 (don’t get an off brand).
I would also add an xacto knife set;
the cordless dremel moto tool (make sure it’s the one with lithium battery [10.8 volts IIRC]);
needle nose tweezers (get a good set and make sure the tips close without a lot of pressure);
small screwdrivers (slotted and phillips - get a set.);
pin vise with ball handle;
set of tiny drill bits with indexing case;
good paint brushes for painting and cheap brushes for plastic cement;
small anvil;
small pliers with smooth jaws and blunt end;
small file set (one for plastic, one for metal);
emery boards;
NMRA/NASG standards gauge for your scale - real important, check everything with it;