Potters Bar Inquest

[Zoe]

These are only guidelines. Police can still book you for doing 0.1 mph over the limit.

 

[Failed Unit]

probably best to continue this here http://railforums.co.uk/showthread.php?t=33344. Sorry mods if I am oversteping the mark!

 

[anonymous0101]

i think speedometers in trains can still under-read or over-read in the same way.

 

[jon0844]

In a car, tyre pressure can affect the speed reading - as can swapping the wheel for one that is a different size, with a low-profile tyre etc.

On a train, I presume there's little to alter the reading if it's done on a metal wheel revolution? I guess if it slips that would mess things up, but only for a fraction of a second.

Do modern trains also use GPS to measure speed (even as a secondary option)?