There's several tales in the Maine two foot books about crews running (on the flats anyway) at 60 miles per hour on passenger trains. SR&RL, WW&F, and maybe even B&SR. However the ride was pretty rough in the cars behind the engine!
Passenger engines are counterbalanced differently than freight engines, of course. Maine 2 foot engines are smaller compared to the South African engines - thus they weigh less. Maybe weight makes a difference in this case.
On the WW&F one time we did a speed test on an engine and got up to 40 MPH. I guess the ride in the cab was pretty rough.