But on top of that, trains don't have to worry so much about whether or not their speed - or lack of it - impedes other traffic.
Actually, they do. Not so much the drivers, but the planners certainly.
An example I used elsewhere was to consider a two-track line (one track per direction) 20 miles long, say between two sections of four-track line. Passenger trains can (in theory) follow each other at 2-minute intervals at 120mph. Freight trains can follow each other at 5-minute intervals at 60mph. This is normal spec if the signals are about a mile apart.
Now consider if you mix passenger and freight traffic on that line. Say a passenger train goes first, immediately followed by a freight train (starting 2 minutes later). The passenger train will arrive after 10 minutes, the freight train 12 minutes later than that. The next train can't reasonably arrive until 5 minutes after *that*, so if it's a passenger train, it should aim to start a whole 17 minutes after the first passenger train left, in order to avoid being held up.
But you could have fitted 7 more passenger trains in between these two, if the freight train were not there. Railway planners therefore say that the freight train "consumes seven passenger paths".
A second freight train can of course start on minute 19, 17 minutes after the first freight, and will not be held up. But another 2 freight trains could have been fitted between them if the second passenger train were not there. So the passenger train, likewise, "consumes two freight paths".
Overall, the efficiency of a track with different speed trains on it is much lower than one where all the trains run at about the same speed. That's why railways are so often built with four tracks instead of two, so that fast traffic can have a line to itself, and so can slow traffic.