When I first started taking by bike by train, in the seventies, it had to travel at child rate and railcards hadn't yet been introduced. I had to fork out a lot of money but getting it on the train was easy as almost every train had a large luggage area designed for transportation of parcels.
Now it's free, but capacity is drastically reduced while demand has soared. So if you're planning a trip into an unfamiliar area you need to be aware of the local situation or you may spend a lot of time hanging around stations. If you're planning to start commuting by bike/train you'd be better off making enquiries locally - talk to the train operating company and to regular commuters. This section is aimed at cyclists planning a holiday.
It's usually a good idea to pre-book and in many cases it's mandatory. Capacity is dependant on type of train and to a lesser extent on the operator, some trains being limited to two and some operators banning bikes on peak hour services. The best online sources of information are the National Rail site which is operated on behalf of the train operating companies and the A to B Magazine site.
The guard is responsible for the safety of the train so can refuse to let you board if it's so full that your presence is likely to cause discomfort to other passengers - in practice this rarely happens if you have a reservation. If you miss your train you'll be dependant on the guard's powers of discretion to allow you on the next one without a valid reservation. I've never been refused - most guards are reasonable people doing a difficult job and genuinely want to help cyclists. If you adopt an aggressive attitude you may not fare so well!
The National Conditions of Carriage stipulate that bikes must be labelled with their destination station and must not be locked to a fixture of the train - in an emergency it may have to be moved.
It sounds a lot harder than it is. I've taken my bike by train a hundred times or more, missed connections, decided to come home early and never once come unstuck. Knowing your trains, how the rail system works, being able to read timetables and devise routes helps but isn't essential.
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