Commuters dogged by cancelled Thameslink trains will not be surprised to hear that they run along one of the worst tracks in London, according to the director of a transport watchdog.

Rufus Barnes, director of the London Transport Users' Committee (LTUC), said: "Thameslink and Midland Mainline are our problem operators. People complain most about those services."

Commuters taking Thameslink trains through Barnet last week (December 15-19) had to put up with delays, fewer trains and a service that did not even stop at stations in the borough. An emergency timetable was in operation on Christmas Eve because of signalling problems at Kentish Town.

A Thameslink spokesman said: "We have had a horrible week as far as train failures are concerned. Door problems, signal problems and vandalism on the line.

"But we have also had point and track failures and unfortunately that leads to cancellations and fast-running services and that means stations between London and places like St Albans, Luton and Bedford do get missed out quite a bit. We are very dependent on the infrastructure."

If a Thameslink train is delayed for any reason, it will not stop at stations between Cricklewood and Elstree & Borehamwood, in order to ensure that trains behind it will be on schedule.

Mr Barnes agreed this was the only solution.

He said: "They have the tacit support of the Strategic Rail Authority and however disruptive it is for passengers at the time, it is in their overall interests to do this.

"But Thameslink have a lot of trouble with the infrastructure, operated by Network Rail, and we do want to see Network Rail improving the infrastructure so they will be able to get their trains running on time."

A spokeswoman for Network Rail, which took over responsibility for the track from Railtrack when it went into receivership in October 2002, agreed improvements did have to be made but it would be years before anything happened.

She said: "Our chairman, Ian McAllister, has said it would be between three and five years before any significant changes would be made."

Network Rail replaced 950 miles of track last year, and will be taking care of all maintenance work as of next year, rather than hiring outside contractors.