I have high-speed trains in my network that are slowing down but not stopping at the station. Then the trains disappear at the end of the link. Why is this happening and how do I fix it?

The train cannot stop because its deceleration capability is limited and thus it needs a longer look-ahead distance than normal traffic to see the stop in time. If it is unable to stop in time, it leaves its route and the train is removed from the network (this event is logged in messages).
To fix this, in the driving behavior parameters used for the train, increase the maximum look-ahead distance.