What sets the speed in CTRL tunnels through London?

"Michael Bell" <michael@beaverbell.co.uk> wrote in message
news:cb39ffd94f.michaelbell@michael.beaverbell.co. uk...
> Eurostars go through Kent at 180 mph, so we are told. Then they slow
> down to 14 mph in the CTRL tunnels in London. Why?
>
> Is 140 mph simply "the next notch down" on the signalling system?
>
> Or is this limit set by air resistance in the tunnels?
>

Going from 180 to 14 is a big drop!

McK.

Mr.Bell in his original question made a typo of "14" when any intelligent
person would know that he meant "140". Can we proceed on that basis and
perhaps someone with the technical expertise could offer us all the correct
answer.

MJW