Train company bosses have denied ordering guards to charge the maximum fare if passengers are unable to buy tickets before boarding.
SWT said train guards could still use their discretion over fines
South West Trains (SWT) has denied reports it has been forcing thousands of passengers to pay penalties even if they were delayed by long queues.
A SWT spokeswoman said guards would still listen to people's explanations and make an on-the-spot decision.
She added that a pager system alerted train crews to queues at stations.
"There has been no change in our policy. Discretion is allowed where needed," she said.
"We do want people to purchase their ticket before they board their train, that is absolutely true.
"However there are occasions where discretion needs to be used."
Those occasions would include instances where long queues had built up because of a large number of passengers or because of staff illness.
Maximum peak fare
Currently, station managers alert crews by pager if there are long delays and guards can also phone them to check passengers' claims.
She also pointed out the company's network had 259 vending machines, with another 194 due to be in place by September 2008.
The Times newspaper claimed on Monday that the company had told its guards not to accept any explanation from passengers and to charge the maximum peak fare, which is often double the normal fare.
South West Trains has come in for criticism recently for increasing fares on some services by up to 20%.