I don't think you can have too many of either. Track Markers are just addresses and unless you have a command (Drive/Navigate via/to) one of them, they are just pretty colored pointers. Track Direction Markers can cause confusion if they contradict each other. Since each one affects all of the track between two junctions, a Direction Marker can affect a junction choice very far away. It is easy to make each junction choice a No Entry track because a distant Marker is against you. That is what they do, prevent a train from entering a track if the pointy end is facing that train. They will not force a train to take a specific track. If there are two tracks, both with a Direction Marker pointing in the right direction, other rules will apply; Track Markers if they exist or the path with the shortest distance, etc.