You operate both cargo and passenger trains, adhere to timetables, control and maintain speed, adapt to different weather conditions, avoid wheelslip, etc, etc. There's different scenarios you can do which range from Easy to Hard, and can be anywhere from 20 minutes to 4 hours long in real time.
The learning curve really isn't that big. Most of the challenge is getting used to the "feel" of different locomotives, how they accelerate and brake. That sort of thing. The physics are surprisingly good; when you're sitting in the cab and going around a curve, you can actually feel the train tilting every-so-slightly and bumping along the track.
It can be a very relaxing game (as long as you pick an easy scenario). There's something very calming about sitting in the drivers seat of a passenger train, hearing the engine roar beneath you, and watching the rainy European countryside as it rolls by.