Attempts to cruise above 200 mph produce a tremor strong enough to throw trains off their tracks. Called "Critical Track Velocity," this phenomenon causes rails to vibrate and buckle dangerously. British engineers consider Critical Track Velocity to be the steel-on-steel equivalent of the infamous sound barrier that tore apart early jet fighters and still limits the speeds of commercial airliners.
CTV is a major reason why China won't allow its high-speed trains to travel more than 185 mph. In Britain, engineers are working on the CTV problem, but they don't expect a quick answer. Meanwhile, the French allow their TGV ("Train à Grande Vitesse" which translates, oddly, as "Train to the Big Fastness") to exceed 200 mph and keep their fingers crossed.