The action by the Office of Rail and Road comes after an earlier investigation found last month that “no-one took charge” during the fiasco.
The organisation has now said it will inquire whether Northern and Govia Thameslink Railway, the operators most affected by the chaos, did “everything reasonably practicable to provide appropriate, accurate and timely information to passengers”.
The move was welcomed by the watchdog Transport Focus. Its chief executive, Anthony Smith, said: “Passengers were badly let down when new timetables descended into chaos on some Govia Thameslink and Northern routes. One thing that would have helped passengers work around these problems was accurate, timely and useful information – but in many cases that was not provided.
“So it’s only right that the regulator investigates if these companies breached the passenger information obligations in their licence.”
The Office and Rail and Road says it wants to see whether Northern breached Condition Four of its “Statement of National Regulatory Provisions”, which requires train companies to provide enough information to enable passengers to plan their journeys “with a reasonable degree of assurance, including when there is disruption.”
Northern has been given until next Friday to respond. A letter from the regulator to the company’s managing director, David Brown, warns that the investigation could result in “formal enforcement action”.
Stephanie Tobyn, a deputy director within the ORR, says in the letter: “We have identified concerns with Northern Railway’s provision of passenger information”.
She says a decision will be made before the end of next month on whether a breach occurred.
Last month’s report ruled that Network Rail, Northern and the Department for Transport “had all made mistakes”.
Following its publication, the Government promised a major review of Britain’s railways.
The botched timetable change in May was the biggest in a generation, involving changes to 46 per cent of train times. But it led to wholesale delays and cancellations across the North and outrage from travellers and politicians.