Doing a quick Google search I found a Discord and Reddit community, I would start with them and ask around to see if anyone has modded the program.
If you were open to a different sim, you might want to try Trainz. I used to be a content creator way back when, and it is a very easy program to 'mod'. There's a massive community that creates both freeware and payware content, including several groups that do commission work.
Even further back, in my Microsoft Train Simulator days, I do remember some freeware Silverliners were made for that sim. Check out the Train-Sim website's freeware downloads, they should be there. MSTS still has a very large community for such an old sim.
"That sapling that once grew just south of Wassaic may be long gone, and the Harlem Line’s appearance may have changed over the years, but for decades to come, I can count on it continuing to provide me with funny recollections"