Oh, there's lots of slow-speed running out there, because the track conditions in many lightly-used, old, worn-out branch lines won't handle anything else. Have a look at the table of FRA track classifications. Passenger and heavy freight lines will be class 4 and 5; lines like AERC are class 1 and 2 when they aren't "excepted track" where trains have to move at a walk or risk bouncing right off the rails.