• andyp posted a link, but here's what eurostar say about taking bikes on trains:

    If you can fold or dismantle your bike and place it in a bike bag with the saddle, handlebars and wheels removed, you can carry it on board yourself as part of your luggage allowance as long as the overall size is no bigger than a normal suitcase.

    Alternatively, you have 2 other options.

    From 7 April you will be able to reserve a place for your bike on the same train that you are travelling on. (This is subject to space being available). This service operates on our London, Paris and Brussels routes only and will give you added assurance of knowing that your bike is travelling with you.

    You can make a reservation or find out more about this service by calling us on 08705 850 850 or visit the EuroDespatch Centre in person at St Pancras International. Charges are £20 one way and it’s necessary to quote your Eurostar reference or show your ticket.

    Or you may simply wish to use our registered baggage service which also operates between London, Paris and Brussels Eurostar terminals as well as Lille Europe but does not guarantee your bike travels on the same train as you. We will do our utmost to send your bike on the next available service, space permitting. However, our guarantee is that your bike will be available to collect at your Eurostar destination station within 24 hours after registration.

    But this service also allows you to send your bike in advance of your journey and then it’ll be ready to collect when you arrive. If not, you can still turn up on the day. So if you are able to be that little bit more flexible, this can also be a useful option.

    Charges are £20 and it’s necessary to show your Eurostar ticket or booking reference when you register.

    For more details about our registered baggage service take a look at the baggage information or feel free to call us on 08705 850 850.

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