On a crowded HS1 train from London once I spent the entire trip with my Brompton on my lap. I didn't have a lock, and wasn't prepared for it to be out of site, out of reach, or on an over head rack. Not the most comfortable trip I've ever had.
Dutch trains, and some Belgian ones conveniently have back to back seats with a space between them where a Brompton fits nicely. If all this is full, there is enough space under the seats for a Brompton on it's side...
The Dutch VIRM double deckers, if you go up stairs, there is a very convenient Brompton sized space just to the left of the door you come through to ender the carriage.
On a Eurostar it fits nicely in the luggage rack, which is heavy duty enough to allow you to lock the bike to it quite nicely. This seems to apply to Thalys and TGV stock too.
J