If you catch a train on the Ashford to Tonbridge line on a Sunday (the one an hour which stops at Hildenborough instead of Pluckley), they can almost certainly reach 100mph between Ashford and Headcorn. Using a GPS based speedometer on my iPhone, I noted 98mph as the top which, given its inaccuracies, could easily have been 100mph.
Needless to say, the good old
08:53 Charing Cross to Ashford would no doubt be a good demonstrator of capable speed (Waterloo East, London Bridge, Tonbridge then fast to Ashford!). A train I wish I could have the opportunity of riding!