You may bring a folding bike or a kid's bike on board as long as it isn't longer than 85 cm long and therefore is carried in a protective bag that covers the entire bike. This is done to protect your bike, other people, and their luggage while at the station, on the train, and while going through security.

If your bicycle does not meet these requirements, you should consult Eurostar to reserve a seat; all of the necessary information is available in their website.