Some train companies are "thumbing their noses at passengers" by not paying out for costs incurred when services fail, says Which?
Passengers are entitled to claim if the last train of the night is cancelled and they have to get a taxi instead, says the consumer rights group.
But it found 12 out of 26 rail firms gave incorrect or inconsistent advice to callers asking about compensation.
Which? said train companies persisted in skirting the law.
It described the findings as "another indictment against train companies failing to act in the best interest of their customers".
"The rail regulator regulator (the Office of Road and Rail) must now step in and take enforcement action against companies that persist in misleading customers on consequential loss – and train operating companies must start to advise and process passengers' claims fairly," it added.
Which? made two "mystery shopper" calls to each of 26 rail operators, asking whether an elderly friend or relative could seek compensation for having to get a cab home when the last train was cancelled.
Six firms – Cross Country, Grand Central, Greater Anglia, Heathrow Express, ScotRail and Stansted Express – consistently and wrongly said no, Which? said.
Another six companies – Arriva Wales, Chiltern, Southeastern, Thameslink/Great Northern, Virgin West Coast and West Midland – gave inconsistent advice, it added.
"When we put our various findings to these companies, ScotRail, Greater Anglia, Stansted Express and Grand Central said that they do cover consequential losses, which suggests that their individual staff members are not articulating company policy correctly," Which? said.
"Heathrow Express also confirmed that they intend to retrain their staff. Cross Country did not respond to our request for comment."
In response to the findings, Greater Anglia and Stansted Express told Which? that they would not generally cancel a last train, but that if they did, they would organise replacement buses and taxis.
Grand Central also said that it would provide alternative travel or, in the most extreme cases, hotel accommodation.
ScotRail said it was grateful to Which? for "highlighting this potential training issue", while Chiltern said it had "already made changes to improve clarity". Heathrow Express said it would retrain staff.
However, Govia Thameslink Railway said it did not accept the validity of the research, as "the sample size of just two calls is too small to validate any conclusions and we have had no sight of any of the data".