As in the thread, I saw that my train was being pulled by a Dash 8 and a Genesis, but that got me to thinking, what is the oldest Current passenger locomotive in revenue service? I thought it might be NJ transits GP40s but I feel I am wrong on this.
You probably need to specify in commuter and/or amtrak service.
There are steam engines pushing a hundred years old in tourist/museum service ...
and lots of first generation (GP9) diesels, too