Many rail operators have unfortunately banned cycles on trains at peak times, and LCC is campaigning for better provision for cyclists on the rail network.
The current restrictions change too frequently to be covered on this page, but the A to B magazine website has a comprehensive list of train restrictions which is updated regularly. You can find this by following the link on the right.
You can also pick up a leaflet on traveling with your bike at most railway stations, or download a guide to Cycling by Train from the National Rail website (see link on the right).
Folding cycles can normally be carried on any service, though if they are in a case or wrapped up they may be looked on more favourably. On some services you can take full size cycles but only during off peak hours. It is worth checking with the service operator for their peak times and restrictions.