South West Trains has, according to reports, told its guards to impose penalty fares on passengers who tried to buy a ticket at one of its stations, but couldn't because of long queues at windows or ticket machines. Are these penalty fares legally enforceable? Almost certainly not.

A confidential memo, seen last week by The Times newspaper, suggested that South West Trains is planning to introduce a system under which guards are judged according to the amount they collect in penalties.

The memo, headed "commercially sensitive, please do not circulate", instructs guards to treat passengers as fare dodgers even if they ask to buy a ticket on the train.

As well as being bad for customers and guards alike, the policy is legally dubious. Rail companies have to rely on the penalty fare rules 2002, made by the then Department of Transport, to levy such charges.

These rules are explicit. A penalty fare may not be charged if there were "no facilities to issue the appropriate ticket". This, at least arguably, means there must have been a window at which there is no queue. In plain English, a person is not available if he is serving a queue. Nor is a machine available if it is in use.

SWT says its policy is to sell a ticket within, at most, five minutes of waiting. Although that does not tie in exactly with the concept of "availability".

In an attempt to get around the problem, the train companies have come up with "conditions of carriage". These don't incorporate the rules' wording, but say a penalty fare is payable if there is no window open and no working machine. It is doubtful that a passenger who has bought no ticket, and hence made no contract with the rail company, could be subject to any conditions. The conditions are invalid if they do not follow the DoT's rules.

SWT's website says one of the reasons for imposing penalty fares is to deter those who do not buy a ticket until challenged. But proving the intention to evade, beyond reasonable doubt, would be virtually impossible. A penalty then seems fair, but that is very different from a ticketless passenger seeking out the guard.

Those who couldn't buy a ticket should politely refuse to pay the penalty. The guard is entitled to a name and address and to know where they got on, and will get off. Mentioning paragraph 7.4 of the penalty fare rules is likely to win the argument, at least on the train.

If the guard issues a penalty notice anyway, there is 21 days to appeal to the company. However even if there is no appeal, or the appeal is not allowed, the company is not automatically entitled to its money. It first has to sue in the county court. Judges hearing such claims would not give judgment for the penalty sum unless the company could justify it.

There has been no reported case of a train company suing in this way. The last thing the rail industry would want is a pronouncement by a judge on its levying of penalty fares.

? Richard Colbey is a barrister