Gutmann last month secured legal approval to bring a collective action against Govia and two other rail operators - the South Western and Southeastern rail franchises - seeking compensation worth up to £93 million.
The claim alleges that passengers were not given sufficient access to so-called ‘boundary fares’ – under which travellers holding a London Travelcard should be offered discounted tickets taking them from the outer boundary of any zone covered by the card to their ultimate destination.
It effectively stops London commuters from being charged twice for overlapping stretches of the same journey.
The companies are alleged not to have not made the ‘boundary fares’ sufficiently available for Travelcard holders to purchase, nor making passengers fully aware of their existence.
Some 240 million journeys since November 2015 could have benefited from boundary fares if travellers had been aware of them, it is claimed.
A GTR spokesperson said: “We are aware of this proposed claim. Should the claim progress, we will make our submissions to the tribunal in due course.”
Great Northern serves destinations including Cambridge, Peterborough, King’s Lynn and Ely while Thameslink is a key commuter line to central London linking Brighton, St Albans, Bedford, East Grinstead and Luton Airport.
Southern serves destinations including Brighton, Hastings, Portsmouth, Southampton, Eastbourne and Milton Keynes.
Retired Gutmann, who spent eight years working for London Underground before becoming head of research for Citizens’ Advice, has jointly instructed law firms Charles Lyndon and Hausfeld & Co in the claim.