Trains slowing down and lagging when going up hills.

When going up hill I expect my trains to take it normally while still going fast but instead my trains glitch and it happens every time they take corners and tracks going down perfectly so I don’t understand why my trains do that. So first go in a world then make tracks on top of a hill. Second place the train then make a corner track. Last see how the train takes the corner track and then see how the train takes the hill track.