The only thing dumb is to NOT ask a question. Nobody knows it all and we all had to learn somewhere.
Train service usually refers to the crew operating the train. In traditional terms that would be the engineer and conductor. It used to include brakemen, flagmen and firemen as well. Now days some places do and some don't use brakemen. Another relatively new position is a utility man. Some places, that's like a traveling brakeman who works with more than one crew one after another, other places he (or she) may work part of the time with a train crew and then perform other categories of service.
Other categories would be M/W - Maintenance of Way, or in layman's terms the folks who maintain the track. M/E would be Maintenance of Equipment. Clerks, supervisors, brass (officials) - there are probably more but that's what comes to mind.