Original sentence: A prototype of the HST, the British Rail Class 43 (HST) achieved 143 mph on the line in 1973 before current UK legislation requires in-cab signalling for speeds of over 125 mph which prevents the InterCity 225 train-sets from operating at the Class 91 design speed of  in normal service
Edited sentence: A prototype of the HST, the British Rail Class 43 (HST) achieved  on the line in 1973 before current UK legislation requires in-cab signalling for speeds of over 125 mph which prevents the InterCity 225 train-sets from operating at the Class 91 design speed of  in normal service