Seven train operators are not using a system available for eight months to automatically notify passengers when services are cancelled or changed, a regulator said.
The process involves an email or text message being sent to online ticket buyers when services are amended up to 48 hours before travel.
It is available to train operators, independent retailers and travel management companies, but not all businesses have signed up despite journey reliability sinking to a record low.
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Rail regulator the Office of Rail and Road (ORR) said the operators not using the automatic notification tool developed last year are: Hull Trains, London Northwestern Railway, ScotRail, South Western Railway, TransPennine Express, Transport for Wales and West Midlands Railway.
It wrote to the companies stating that “passengers reasonably expect train operators to be proactive in keeping them informed” and they are expected to begin using the system by the end of March.
Sixteen operators are using the system.
For changes made within 48 hours of travel, the regulator has asked operators to make it simple for passengers to sign up for automatic alerts.
ORR director of strategy, policy and reform Stephanie Tobyn said: “Letting people know of a change to their travel booking well in advance is an important first step in improving passenger information, and it is disappointing that not all passengers are benefitting from a system that has been in place since May 2022.
“Train companies need to remain consistent with the passenger information licence condition which requires passengers to be able to plan their journey with a reasonable degree of assurance.
“So there should be no further delay in companies putting this system in place now.”
A spokesman for industry body the Rail Delivery Group said: “As the Office of Rail and Road makes clear, many passengers are now benefitting from the new system to keep passengers automatically updated about their booking.
“The train operating companies that are still to implement it are committed to doing so.
“Regular and timely information that helps passengers to plan their journeys is an important part of their overall experience.”
ORR figures show the equivalent of one in 13 services across Britain was cancelled during the four weeks to January 7 amid strikes, staff shortages and severe weather.
That represents the poorest reliability in records dating back to April 2014.