Complex sentence: An A4 with the same load on a "good run" would be doing about 50 to 60mph at the summit of Stoke Bank; on a run on 8 September 1961 "Mallard" had its train travelling at 78mph.
Split into simpler sentences: An A4 with the same load on a "good run" would be doing about 50 to 60mph at the summit of Stoke Bank. On a run on 8 September 1961 "Mallard" had its train travelling at 78mph.