Going on holiday to Monaco with your bike is pretty straight forward, but as with a lot of things, its better to book in advance.
The Eurostar allows you to take your bike on the train, however regulations stipulate that it must be carried in a bike bag or box.
"Passengers with bikes have and continue to be important to us. Our new policy has been introduced so that we can use the space on our trains more flexibly, by carrying the same or more bikes depending on the demand from passengers. The only change is that bikes will now need to be carried in a bike box, either your own or one which we are happy to provide. When packaging bikes in this way, they take up less space which means that we can carry more bikes, or any other type of luggage."
If your bike is placed in a suitable bike bag or box that is no longer than 85cm, you can carry it on board as part of your baggage allowance for ALL journeys from London. For bike bags or boxes over this size then you will need to use the registered luggage service. This costs between €25 - €29 one way from London to Paris, Brussels or Lille. This service is not permitted on direct trains from London to Lyon, Avignon, Marseille and the French Alps.
They will do everything they can to ensure your bike travels on the same train as you. But if it can’t, they guarantee that you will be able to collect it at your Eurostar destination station within 24 hours of your departure. Your bike will generally travel on the same train as you unless there is not enough space. If this is the case, they will send it on the next available train.
Once you're in Monaco getting around with your bike can be straight forward, but again planning ahead and getting to the station early are highly recommended.
Domestic or regional trains (TER) and Tourist trains quite often allow you to transport your bike for free, although you may have to put it in a bike compartment, or hang it from a hook to save space, or fold it up and take it on as hand luggage. Spaces are limited and you cannot reserve in advance, and in some regions bikes aren't permitted Monday-Friday during working hours as the trains are too busy with pedestrians.
Regional trains in Ile-de-France, the region surrounding Paris, are called Transilien. These are the country’s busiest commuter trains. They operate into all of Paris’ stations and run on lines with a letter associated (line K, line P, etc.). They also include the RER trains, which run on lines A to E. You can take your bike on board a Transilien Monday to Friday, before 6.30am, between 9.30am and 4.30pm, then after 7.30pm; At any time on weekends and bank holidays and at any time if it is folded or placed in a bike bag and does not cause fellow passengers any inconvenience.
On Intercity trains it's best if you can dismantle your bike and carry it on in a bag no more than 120x90cm. Some of these trains require a seat reservation, whereas others do not. Consequently, according to the type of train, it may or may not be necessary to book, or pay for, a place for your bike.