Dr Wayne has it right. The decals are almost certainly covered with DullCote or a clear lacquer. I don't know anything that will cut thru lacquer without cutting thru the underlying paint.
I have had good luck simply brush painting over the old decals and redecaling on top of the fresh paint (after it dries of course). With a good color match the paint will blend right in and become invisible. I used Floquil B&M blue on a set of Bluebird GP38s and the Floquil matched the factory (Athearn) paint perfectly. On steamers, black is an easy color to match well.
David Starr www.newsnorthwoods.blogspot.com
You don't mention whether the locomotive is diesel or steam, but chances are that either will have a clear finish applied over the decals, which means that it will be difficult to avoid marring the finish.I'm in the midst of painting or re-painting a number of brass steam locomotives for a friend, and while the painted ones are all lettered for the right railroad, I'm not much impressed by the paint jobs or the excessive weathering. I could simply paint over everything, but I chose to remove the lettering, which appears to be mostly decals and perhaps some dry transfers. For a really poor paint job, I'd disassemble the whole thing and put it in lacquer thinner for a complete stripping of the finish.
I've found that methyl hydrate works well to remove most clear finishes and the lettering beneath it (put a little on a rag and simply wipe, repeating as necessary - wear nitrile gloves, as methyl hydrate is easily absorbed through the skin) but the locomotive will likely require repainting or at least extensive touch-up.If your locomotive is a steam engine, most of the lettering will be on the cab and tender, making the task much easier. Strip off the lettering as mentioned, then simply re-paint only the cab and tender. When steam engines were still in regular service, those that were kept relatively clean were not completely flat black paint. Even if I'm painting a locomotive for someone who wants it heavily weathered, I use several different versions of black on various parts of the loco and tender, then apply weathering over that. The areas least affected by heat are the cab and tender, so the paint there is more likely (with proper care) to remain as it was when first applied.So, if you generally like the current paint job, simply repaint those two areas with a black which complements the paint used on the rest of the locomotive.
For my own paint jobs, and those for the friend mentioned, I use several different blacks, all mixed using black, grey, and red, in varying proportions.Here's one of my own, recently modified and painted. It's meant to represent one recently outshopped, and has not yet been weathered - it won't get much, as I don't want it to look the same as ones that have been in service longer...
If your locomotive is a diesel, you may be able to do a selective repainting, depending on the layout of the lettering and the paint scheme on which it's applied, but I doubt that you'll escape painting of some sort once the lettering has been removed.
Wayne
I've removed decals and lettering from a number of different plastic models using solvaset and other decal removing agents. The techniques always damage the underlying paint some but the re-decaling and weathering hides any damage.
I have a brass locomotive that was custom gloss painted and lettered with decals. Anyone have any suggestions about methods to remove the decals without destroying the underlying paint job. it may not be feasible. A google search didn't find much. Thoughts?