Trains going over 800 KM/h risk bugging out (ignoring or being ignored by) train signals under certain circumstances. I don't know the exact values or circumstances, but be careful. It has to do with the tick rate of the game and how quickly a signal can check, change state and train reading signal states. There's no fix for this glitch.
My advice is to keep adding cargo wagons until the train can no longer go above 800 KM/h or use a slower train.
