From your numbers, the differences in speed are very small. 10 miles-per-hour (full scale) is 14.67 feet-per-second (full scale), which means the model engine is moving over the track at 0.306 feet-per-second (about 3.6 inches-per-second). You ran the engines for a distance of 60 feet, which would take 196.4 seconds - about 3 minutes and 16 seconds - a long time for a difference in relative position of only 2 feet. That would be (2 feet in 196.4 seconds) 0.0102 feet-per-second on the track - equivalent to 0.489 feet-per-second at full scale - which is 0.333 miles-per-hour at full scale. So, if I'm interpreting your numbers correctly, the slower engine is running at 10 scale-miles-per-hour, and the faster one would be at 10.333 scale-miles-per-hour. That is not a large difference in speed between the two engines - just slightly more than 3 percent. That should be a sufficiently small difference in speed between the two engines such that you could run them coupled together.
10 scale-miles-per-hour is a low operating speed, at which small differences in internal friction probably have a larger effect than at higher speeds.
MELGAR
