Why not instead of a speed limit, have a wall that goes up (like the concrete wall doors you can put between walls, that automatically go down when you walk into them), when a train is coming? So that you can't walk through the path when there is a train coming. Perhaps a signal that closes the gate so you can't go through until the back of the train has passed the next signal that opens the gate again.

This would be safer than slowing down trains, it keeps your network running optimally at all times instead of slowing down trains uselessly at all locations even if you aren't around.

You can connect rail signals to the gate using circuit network to get the desired behaviour. No need for mods in this case.

Anyway trains going with 200 km/h in places where players are performing some maintenance tasks (cargo stations etc) are not so safe and you are not able to create such rail crossing. From my game experience it follows that slowing down trains on some small segments doesn't change anything (if signs are placed wisely).