Have a look at this page : https://www.seat61.com/bike-by-train.htm

Eurostar will be your biggest issue. Every country has its own rules.

Honestly it isn't worth the hassle in my opinion. Depending on where you're going roads could be frozen or be fully pedestrian. City centers are also walkable most of the time.

thanks i’ll take a look

A Brompton should be fine on most trains, as it folds down to a reasonable size.

A lightweight bike bag wouldn’t hurt if you can find a suitable one - that way it’s just ‘luggage’ and you can avoid conversation with troublesome staff. 

Good write up here: https://www.reddit.com/r/Brompton/comments/srop3b/my_experience_of_taking_my_brompton_by_eurostartgv/

If it's a folding bike and you can carry it on board yourself, then it's just normal luggage. Folding in and out needs to be done outside the train. In some countries, it also needs to be in a bag or have a cover.

I don’t know any specific rules about folding bikes on train companies. Almost always it’s seen as a bit of luggage, free of charge.

Bromptons are very “foldable” and are easy to fit anywhere where there’s space, can be a nice asset to increase your exploring range when visiting places.

AFAIK There won’t be an issue at all in the UK, France, BeNeLux, Germany, Norway to take your folding bike on trains. For other countries I don’t know exactly, but probably the same: free of charge. :)

Thank you so much everyone. My first post on the community……so helpful