I don't support the UK joining Schengen as it will weaken the security of our national borders, and it wouldn't do anything for the security requirements for the Tunnel either. I would however be unsurprised were the UK an EFTA member within 10 years. It is still "leave" but solves most of the issues.
At risk of going off-topic; but kinda related to Eurostar in a way, the UK government have decided that any non UK/Irish citizens must complete a transit form to travel between The Republic (EU) and the UK - a bit like the ETIAS but coming to the UK. So are we having a hard border and also having Eurostar-style checks as you board a train between the two?
Of course not! So how's it gonna be policed? It isn't! More government Brexit dimwittery of course.
Back on topic, with lengthy security queues (2-3hrs) at Manchester Airport for the forseeable future, Eurostar should be advertising up here in the North West as I'm sure Avanti + Eurostar could get you to Brussels or Paris faster than flying at the moment.
For me - 1.5hr home to Airport + 2hr security queue, spare 0.5hr to the gate, 1.5hr flight + 0.5 through immigration then 0.5hr to the city centres...6-and-a-half hours. Can that be done on Avanti/Eurostar including the change at Euston/St Pancras? How long does security and immigration take on Eurostar post-covid?
Need to take a step back and do some thinking - UK Border Force officials boarding a train in Bordeaux or somewhere (probably require six or so to check the passports, including a senior, and a reserve to deal with any issues). Stopping at CdG or Disneyland to disembark any not correctly documented. Might even need some heavies there too. Won't get any help from the French authorities. Travel back passenger in another train. Probably only do one train in a shift, all on foreign service allowances and board. Probably cost £50+ per passport examined, to be added to the fares. Need a sense check here! Not going to happen.
Two sets of officials required, the French border force and then the British. The French would have to stamp all our passports, not easy on a moving train!
Theoretically, if you have a route A - B - C and A+B are in France and C the UK, you COULD start the checks after station B but you have the problem I refer to. Also if a domestic pax inadvertantly passes station B and has little documentation, how do the officials deal with that?
Pre EU/Schengen I have boarded trains crossing European borders, occasionally officials came on to check, basically a glance at your passport, but in those days they were pretty lax. While Schengen means thay have gone now, the EU/UK border is severely controlled.
Apart from the UK/Irish one of course!