On freight trains, there are always a minimum of two people in the cab. Cabooses disappeared years ago. The conductor used to ride back there. The engineer and conductor are now together up front. The conductor keeps track of the paperwork, communications, and walks in the cold rain at 2:00am to find a broken air brake hose or broken coupler a mile to the rear. The job sucks. These guys get sprayed by skunks, get shot at, you name it. The engineer has a pretty good job. Both guys make BIG $$$.
On Amtrak passenger trains, the conductor checks for tickets and tells thugs to turn down the boom boxes. So he isn't always in the cab. That is what I've witnessed on cross country trains. It may be different in the NE were 100+ mph trains are common. I am not certain, but I think that current federal law requires a minimum of two in the cab now. But only one is an engineer.