Just got back home with the LA Subway, the Red Line. No, the only crew member on board the train is the motorman. He operates the train, the doors and makes all the announcements.
Security is a separate issue. From time to time, you will see security guards, armed, get on board. These are deputized security guards. They might ask to see your valid ticket under normal conditions. Under abnormal conditions they would act as a Cop would.