In TOPS the FFA wagons used have a maximum speed of 75mph, but looking at the timings for 3Q03, Motherwell to Gretna (80 miles) is timed for 59 minutes, while Penrith to Garstang (63 miles) is timed to take 48 mins. This requires an average speed of 80mph. 3Q03 lost 5 mins between Motherwell and Gretna but even so I think that averages out at 78 mph if my (admittedly rusty) maths is correct.
I assume the wagons are allowed to exceed their usual max speed under the test conditions, to at least 80mph, possibly 85/90. Interesting...