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Refurbished trains to boost capacity on the Valley lines have been delayed "until early 2020".

The Porterbrook-built Class 769 trains were due to have been delivered to Transport for Wales (TfW) last spring.

The rolling stock firm said the five trains have been held up by engineering and supply chain issues.

A source close to the company said the project, involving converting older trains to use diesel and electricity, has presented engineering challenges.

Plaid Cymru wants assurances taxpayers will not pay for the cost of the delays.

Porterbrook has apologised and said the trains are "currently going through their final acceptance programme".

A spokesperson said it continues "to actively work with our supplier Wabtec to ensure that these trains are delivered to TfW as soon as possible".

Plaid leader Adam Price said: "With fare payers already at a loss, taxpayers should be given assurances that the contract signed with the train provider has penalty clauses and a cancellation point built into it."

At First Minister's Questions on Tuesday, Mr Price asked Mark Drakeford whether passengers would be compensated for the delays in getting the rolling stock on line.

Mr Drakeford said there would be reductions in fares in parts of the network when Transport for Wales makes an announcement in January.

A spokesperson for TfW, which took over running Wales' train services a year ago, could not confirm when the trains will be operational, but said, "once we have successfully received the trains, they will undergo stringent safety and engineering checks to ensure they are ready for service".

"We will then commence driver training and we hope to introduce the Class 769s in early 2020".

The same trains have been tested successfully in the North of England, where they reached speeds of up to 100mph when in diesel-mode.

Transport for Wales will introduce other trains (Class 153) as well as retain some of the old Pacer trains and Cass 37 loco-hauled trains in order to ease capacity problems while they wait for the new Class 769 trains.

Richard Clinnick from Rail UK magazine said "delays to the rebuilt Class 769s have been caused by supply chain issues as well as engineering problems."

"There is a wider industry issue regarding shortage of rolling stock, and TfW is in the same situation as several other operators reliant on new and cascaded trains.

"These Class 769s were designed to boost capacity, and eventually when they arrive, that is what they will do, but TfW has to get them first."