Like others, done it countless times with mtb and road bike on Eurostar and TGV. I live in London so I ride to St Pancras and head into that alcove adjacent to check-in. Then it’s wheels off, and pull out 4 bins bags and a bit of tape. One wheel in each bag, and a bag at either end of frame. Works fine – every time.
The Eurostar web site says something along the lines of ‘if it can be dismantled to the size of a normal suitcase, we’ll try and accommodate’. On the few times, the E* staff have picked up on it, they’ve been very polite and helpful and directed me to the galley area. Really it’s no problem whatsoever …