A train running at 125 mph consumes 90% more energy than one running at 90 mph. In addition, there are other costs, since the trains have to be specified for higher speeds, one-third of the front and rear vehicles cannot be used for passenger accommodation, there is additional wear and tear, signalling systems must be designed for the longer stopping distances, thereby reducing track capacity, and the railway becomes subject to EU regulations for high speed lines, with all the associated compliance costs.
Reams of careful calculations would need to be made before the proposal
to reduce top speed on some 125mph routes to just under the 100 mph
threshold could be dismissed as lunatic.