TVM is the system I'm most familiar with when it comes to HSR (used on the LGV in France and HS1 in the UK); it has a variable overspeed allowance (5 to 15 km/h depending on current maximum speed). If you exceed that, an emergency brake application occurs.
Japanese ATC and ATS-P, used on all Shinkansen lines and most busy metro/commuter lines, just apply service break until the train is under back speed limit. AFAIK no overspeed allowance.
