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Train operator Northern's six-carriage long services in and out of Leeds station have been delayed by two years, BBC News has learned.

The longer trains were to be introduced at the end of this year but will now not enter service until late 2021.

The delay is because longer platforms at Leeds will not be ready in time

After a disastrous 2018, Northern is under pressure to improve the reliability of its services and deal with overcrowding.

It has now emerged that the much longer services into Leeds on the busiest routes at peak times, which were planned as part of a timetable shake-up for this December, will not materialise until the end of 2021.

But Northern still plans to start phasing in a £500m fleet of new short trains from this spring.

And despite much scepticism amongst passengers, the company still insists it will get rid of its hugely unpopular and rickety Pacer trains, which date back to the early 1980s, by the end of this year.

Why the delay?

It will not be possible to bring the longer services in sooner because several platforms at Leeds station will not be lengthened in time to accommodate the longer trains.

The work is part of a £160m upgrade to the station, carried out by Network Rail, which also includes the creation of an entirely new platform '0'.

Who knew what, when?

Network Rail says it committed to the project back in 2014. Initial estimates said the project would be finished by the end of this year, in time to accommodate Northern's long trains.

However in 2017, Network Rail decided that the redevelopment of Leeds and the longer platforms could not be finished until the end of 2021.

What is now in dispute is when Northern knew that Leeds would not be able to accommodate its six carriage services for an additional two years.

The Department for Transport (DfT) says Northern was told in September 2017. It points out that the change was contained in a report by Network Rail which was published online.

However, the BBC has learnt that a letter was sent to the managing director of Northern a year later, in September of last year.

The letter, from the Rail North Partnership (a partnership between the DfT and its regional equivalent, Transport for the North) also signed on behalf of the Secretary of State for Transport, Chris Grayling, "confirmed" that a list of infrastructure projects, including Leeds station "will not be available for use at the December 2019 timetable change".

It went on to ask what the impact would be on Northern, and how the train company would respond "as a result of the delay of the availability of these outputs".

A spokesperson for the Department for Transport said the letter was a "contractual requirement" and the idea that Northern was not aware of the change at a much earlier date was "simply false".

However Northern believes the decision was only confirmed when it arrived, from its political master, in writing.

Why does it all matter?

The latest figures from the rail regulator, the Office of Rail and Road (ORR), showed Northern's punctuality on its trains fell to an eight-year low in the third quarter of last year.

The delay means long-suffering passengers on services into Leeds will not benefit from much longer trains for an additional two years. The introduction of shorter new trains should not be affected.

And six-carriage services will run from end of this year on other parts of Northern's network, just not through Leeds.

However, the dispute and the sequence of events shines a light on the failings of an overly complicated rail system, which a government-commissioned review is looking to fix.

It makes the case for better integration of the management of trains and rail infrastructure.

It also adds weight to the argument that there needs to be much more regional control.

And it plays into a growing consensus that the vastly complicated, and rigid, franchising system we have today has to change.

All are issues high-up in the mind of the former British Airways boss, Keith Williams, whose rail review will conclude in the autumn.

A spokesperson at the DfT said it was committed to working with Northern to provide an extra two thousand weekly services, creating space for "31,000 extra passengers on Northern trains."