NavList:

Even two mph over the limit couldn't make a difference since they have much larger safety margins built in when they set a speed limit.

I was researching automotive speed control some years ago, and one of the things I noticed was that in accidents involving bends  relatively small increases in speed can make a significant difference in the force required to keep the vehicle on the road, (or I suppose trains on tracks).  This is because the centripetal and centrifugal forces involved increase with the square of the speed.  E.g.  If the recommended speed for a car on a bend is 30mph, taking the bend at 35mph means a 17% increase in speed but a 36% increases in forces required.  With a train, such oversight might cause the carriages to topple outwards off the track or the outside track to give way.  Then as the wreck is brought to a halt the energy to be dissipated (and therefore damage done) also depends upon the square of the speed.  Either way, exceed speed limits with caution.  DaveP