The anytime return between the Isle of Wight and Derbyshire is priced at £501.40 - even exceeding the cost of fares for journeys between Cornwall and Scotland.
Passengers could travel hundreds of miles further and still save more than £30.
The steep cost of fares has been thrown into focus by the latest annual fare increases - which have led to protests by commuter groups.
A 2.3 per cent rise in England and Wales took effect last week, though the hikes passed on to travellers vary across operators.
The 240-mile journey between Shanklin and Buxton was found by Paul Kelly of BR Fares, an independent website designed for experts to search the complex system of ticketing.
Mr Kelly said: “The Shanklin to Buxton fare is clearly an exorbitant price.
“The fare was also the most expensive standard class fare last year, with a 0.6 per cent increase from last year’s £498.40.
“I suspect the pricing is influenced by the fact that it is intended for travel via London and Manchester, and a London to Manchester Anytime Return costs a similarly absurd £338.”
The fare exceeds even the £467.40 price of an open anytime return between Wick in Scotland and Penzance in Cornwall.
James MacColl, of the Campaign for Better Transport, said: “With rail passengers battling with complex ticketing which results in these astronomical prices on some routes, it’s clear the fares system needs fundamental reform.
“This should start with an end to the policy of constantly raising fares.
“But without long-overdue improvements like an end to split ticketing and introduction of equal season ticket discounts for part-time commuters, passengers will still be left with an impenetrably complicated and simply unfair ticketing system.”
The fare also shows how proposals in 2013 to cap the price of a return ticket at £500 proved fruitless, despite a £1,000 first class ticket between Cornwall and Scotland being reduced.
The Rail Delivery Group, which represents freight and passenger operators, is in discussions with the government about how fares it describes as unwanted and unnecessary can be removed from the system.
A spokesman said: “This particular fare hasn’t been bought by anyone in at least the past four years.
“I imagine that anyone making that journey would use cheaper available options.
“For example, if you catch the first train of the day from Shanklin, by the time you reach Euston an off-peak ticket is valid so, using that, the actual cost would be £151.20 - for a ticket which you can just walk up and buy on the day without the need to book in advance.
“The £501 fare exists because the train operator is required under government regulations to maintain a price for a fully unrestricted ticket for this journey - even if no-one needs or wants to buy it.”