This is a vital element of the game. If you want your trains to run as fast and smooth as possible, you need to keep a consistent, flat gradient, so you’ll need more embankments and cuttings. If you want to keep costs down, you can have the track rise and fall over the hills and dips, but they’ll lose speed when climbing even a relatively shallow incline, and will have even more trouble if they happen to stop at a signal or station and have to accelerate from standing uphill.
That’s how real trains work… especially the earliest trains – I’ve heard that older trains could have trouble with anything beyond a 1 or 2% gradient (to the point where they could literally grind to a halt and slide backwards if it was too steep). Hence why real world train lines are usually far flatter than roads, and tend to run along a lot of embankments and cuttings.