Except for drivers limiting their speed, is there a hardware or software limit on the Pendos of 125mph given they are capable of 140mph? I am aware that in the past many locos (I know that a pendo is not a loco) could and did occasionally achieve higher top speeds e.g. Deltics.
Apologies if this has been asked before.
TASS will take control from the driver if there is an overspeed (of 2mph?) above the authorised line speed.
It kicks in where EPS ("Enhanced Permitted Speed") is signed, and always when speed is above the WCML permanent line speed of 110mph.
Once back to the authorised line speed, TASS will then "offer control back" to the driver.
So it's a combination of hardware/software (using the track balises).
The same control system is used on Virgin's Voyagers.
The ATP systems on London-Bristol/Heathrow and out of Marylebone are older and less sophisticated systems but do something similar.
ERTMS as fitted on the Cambrian also does it.
There are no automatic controls on the ECML or MML (yet).
I'm not sure how the MML got away with raising its speed limit to 125mph without having to fit ATP/ERTMS/TASS.
The ECML has "grandfather rights", and will fit ERTMS by 2020, as will the GWML to replace ATP.