Transpennine problems
Mr Merriman also said train operator Transpennine Express' contract is under review.
Transpennine, which operates across the North of England and into Scotland, has been cancelling trains daily for months.
The company blames high sickness rates and a backlog of driver training due to the pandemic for cancellations.
Mr Merriman said that when the Transpennine Express contract comes to end in May, he is "already looking at what needs to be done... with regards to that contract".
He said he had weekly data on Transpennine and another troubled operator, Avanti West Coast, and was monitoring what they were doing to turn things around.
The government recently gave Avanti six months to the end of March, to urgently improve.
A spokesperson for TransPennine Express said on Thursday: "We are committed to the communities we serve and want to assure our customers that we are doing all we can to deliver a train service they can rely on."
Transpennine "continues to work flat-out to deliver higher levels of service delivery and to tackle the issues that are being experienced by customers," the spokesperson added.