My great great Grandpa

There's no giant resource that keeps stuff like that. Google is probably your best bet. When and where did he work for NYC?

You might try the New York Central Historical Society.

https://nycshs.org/

Look for the 'Contact Us' in the upper section and pick out who you want to contact.

Back before my ISP decided I didn't really need a website and summarily deleted it without notification, I used to get many inquiries from people who had ancestors who worked for the railroad I had a short history page about. At the time, there was a specific web page on the Railroad Retirement Board's website to help address how RRB might be able to provide info for genealogical research (for a fee of course). Looking very quickly, I can no longer find such a specific page on RRB's website, but perhaps a deeper search would reveal one.

Note that this would only help if your ancestor worked for the railroad on and after 1934, when RRB was created.

the home page of George Elwood's "Fallen Flags" site
http://www.rr-fallenflags.org
has a link to a Railroad Retirement Board web site. I haven't tried it, but Elwood mentions it in the context of discussing how one might look for information about former employees.