Here is a tip that I’m passing along. Hopefully I not stating the obvious or stealing someone else’s thunder. I tried this last night and it worked GREAT for removing decals. I can’t say for sure that it will work on every decal out there but it worked like an eraser on my P2K Soo SD60s and 60Ms.
I used Microscale’s Microsol on a Q-tip. I swabbed the Microsol on the decal, let it set for about 5~10 minutes and then used both the Q-tip dipped in Microsol and a clean Q-tip to rub the decal off. You may have to use an X-acto knife to get it started. It came right off and with a little clean up you would never notice that there was decal there.
Let me know if this works for you and type (manufacturer) of loco or rolling stock.