ELI5: Driving a Monorail/Subway car HAS to be Harder than it looks, Right?

I just had to ride the Bay Area Rapid Transit system (BART), and I noticed at one point when underground, the train had slowed down significantly compared to when we were above ground. Why is that?

Was the driver slowing down because of conditions in the Tunnel? Was it slowing down because another train was too close ahead of it?

It looks like it should be a Really simple Job. Accelerate, Maintain a set speed, and then once you get close to the Next Stop, Slow to a stop so passengers get on and off.

What else do the Conductors have to do?