Markings on engines optional or mandatory?

[nachoman]

I have no idea, but what an interesting question. I assumed all non-patched locomotives that I saw not on home rails were leased or part of some kind of trade or other agreement. When Uhaul or other truck leasing agencies sell surplus trucks, they usually paint out their own logo before they sell. Same with municipalities. So maybe it is up to the individual selling railroad to paint out their logo if they don't want it to appear on another railroad's tracks.