By Susannah Streeter
Business reporter, BBC News
Ticket offices do not have to offer split fares but cannot refuse to sell them
There are calls for ticket machines at stations to be improved because passengers are often unable to access the cheapest fares.
BBC research suggests that passengers can save almost 40% on some routes if they buy split tickets, or two separate fares for the same journey.
The Lib Dems say machines should allow for the cheapest tickets to be bought.
But train operators say they are designed to deal with the simplest transactions for speed.
Machines only allow passengers to buy tickets starting where the machine is.
The cheapest fare available on the day from a ticket vending machine for the 0800 from Bristol Temple Meads to London is any Anytime Single priced at £74.50.
Passengers can buy a single from Bristol to Didcot for £21.30 and a single from Didcot to London at £24, giving a saving of £29.20, or 39%.
The machine, however, will only sell the first portion of that split fare. Split tickets have to be purchased online or at a ticket offices.
Although staff do not have to advertise them, they cannot refuse to sell split fares if the passenger specifically requests one.
Liberal Democrat Shadow Transport Secretary Norman Baker claims customers are not getting a fair deal.
"It suits the rail companies quite well for people to buy more expensive tickets than they need to through ticket machines.
"People need to be able to contact a real human person and if the booking office is closed, my view is these ticket machines should have a telephone there so you can talk to someone who actually exists and advise you what the cheapest fare is for your journey."
The number of tickets being sold from the new generation of ticket vending machines is growing. Currently, 14% of tickets are sold that way.
But the Association of Train Operating Companies (Atoc) says the machines are not replacing ticket offices and are designed to cope with increased passenger numbers.
0830 Lancaster - Glasgow:
Anytime Single £41.50
Lancaster - Oxenholme £6.60
Oxenholme - Glasgow £23.50
Source: BBC research
''Machines are not designed for complicated transactions, they are designed to move lots of people quickly," said Andy Wakeford, Atoc's head of fares.
"Whether you buy from a machine or from a ticket office, we will sell you cheapest ticket available for the through route from the station you are travelling from to where you are going to. It is not in our interests to make passengers pay more.
"Split tickets are being used in a way they were not intended for.
"We actually go further than other industries and allow people to buy and use them if requested, but we won't go out of our way to encourage passengers to use them in a way that was not intended when there are other good value-for-money fares available."
Watchdog Passenger Focus believes the systems should be upgraded to offer many more fares and reassure consumers.
''Passengers feel like they have to play games to get access to the cheapest fares and that's a real shame," Ashwin Kumar, Passenger Director at the rail consumer watchdog.
"If train companies can sort out the machines, then people will be more confident that they are getting the best deal.''
At the start of the year, rail fares increased by an inflation-busting 6%.