What can happen if I am on a train without a valid ticket?

If you do not have a valid ticket or a valid reason for not having one (as defined in Can I board a train without a ticket?) and you board the train, the train company has a number of options:

The full fare can be charged for your journey
Under the National Rail Conditions of Carriage you could be charged the full fare for the journey. Typical examples of being found aboard a train with an invalid ticket include:  where you have a ticket but have forgotten your Railcard; or where you have an Off-peak ticket but travel in the peak; or where you have an Advance ticket but are on the wrong train. In some instances you may be charged the full Anytime (peak hours) fare; in others it may be the cheapest fare available for journeys made at that time.

You can be issued with an Unpaid Fare Notice (UFN)
If staff on the train consider that your ticket is not valid and that a full fare should be charged you can pay immediately and then, if desired, complain to the train operating company to seek redress. If you do not have the means to pay immediately – or if you choose to complain/appeal before paying – the inspector can issue an Unpaid Fare Notice (UFN). This is not a fine or a penalty but an ‘invoice’ for the fare for the train you are on.
An Unpaid Fares Notice is different from a Penalty Fare.  

You can be issued with a Penalty Fare
Penalty fares were originally introduced to tackle ticketless travel on suburban services whose frequent stops made effective on-train ticket examination impractical. They have since been introduced on other, longer-distance services. They require the passenger to provide a valid reason for not having a ticket. They do not exist on all train routes, and where they are in use clear signs at stations should clarify this to passengers.

If you are in a designated Penalty Fares area and you are not able to produce a valid ticket for inspection you are liable to pay either twice the full Single fare to the next station at which the train is due to stop, or £20, whichever is the greater. Any travel beyond that next station will be charged at the full Single fare.  You can pay the penalty fare on the spot although you do have 21 days in which to pay.  If you feel that you should not have been issued with a penalty fare you may appeal.  Details of the appeals procedure are shown on the penalty fare notice issued to you.

Prosecution for fare evasion
The Railway Byelaws make it an offence to travel without holding a valid ticket and being able to show it on request. A breach of this byelaw is punishable in law and, if found guilty, you would be subject at present to a fine of up to £1000.

An operator can also prosecute for ‘intent to avoid a rail fare’ under the Regulation of Railways Act 1889, s.5 (3) and you may be fined or sentenced to imprisonment for up to  three months. Passenger Focus may be able to help or advise you if you have received a court summons. However, we recommend you also seek legal advice if you are being prosecuted

The industry introduced in May 2013 a Code of Practice on dealing with passengers who either have no ticket at all or whose ticket is invalid.