hau Bill, I think Dave has given simply awesome suggestions for remediation of "railroad land".
Odds are that the oil slicks are diesel fuel from leaking fuel tanks on locomotives (I've witnessed it literally pouring out of some both in the yards and because of derailments).
First thing to do would be to get the
fungi growing, (there are specific "oil eating" bacteria that are used for major spills, I'll see if I can get you some info on which and how much they would cost if you like).
Redhawk