I've run train much faster than that, and over worse track. In fact, there is an advantage to running fast. On the Lehigh Valley RR's line over Penobscot Mountain in Pennsylvania, there were timetable special instructions that said that if you could not maintain a speed above 30 miles per hour, you had to reduce to 10 mph. This was because with stick rail (with joints...not welded rail) there is what is known as a "critical speed." Critical speed on std. gauge lines is around 14 mph, and this is a speed where the rocking of cars will set up an harmonic motion that will quite literally rock a train off the tracks. Below 10 mph and above 30 mph, you don't develop the bad rocking of the cars. Back to the LVRR, I used to run piggyback trains at 60 mph on tracks where you couldn't see the tracks for the weeds, and the piggy back cars rocked so badly the wheels lifted off the rails. You learned to never look back at the train!
Keith