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A new faster train set to travel between south Wales and London caused commuter mayhem on Monday – thanks to a fault on its first outing.

Transport Secretary Chris Grayling and Welsh Secretary Alun Cairns were both on board when the Hitachi bi-mode Intercity Express Train – which passengers have been promised will be a faster, more modern train, with increased capacity, greater comfort and better connectivity – was forced to stop.

Hitachi said the service had experienced some "technical challenges".

The new hybrid trains, which have a top speed of 125mph, are designed to replace the existing InterCity 125 trains as part of the UK Government’s Intercity Express Programme.

But there were problems on its maiden outing when the first service from Bristol Temple Meads to London Paddington broke down in Taplow, Berkshire, causing massive delays.

The 6am service from Bristol blocked the fast lane due to what engineers called a “minor technical fault”.

After leaving Bristol 25 minutes late due to another fault the train left Reading where it was set to switch from diesel to electric overhead wires at Maidenhead.

But it came to a grinding halt, leaving Mr Cairns and other executives stranded on board until the problem was fixed.

After a total of almost two and a half hours the maiden service arrived in the capital 120 miles away – averaging 50mph on the trip – more than 40 minutes late, which would see normal commuters entitled to compensation.

The first service back to south Wales – the 8.15am to Cardiff – was then cancelled.

A Hitachi Rail Europe spokesman said: “Great Western Railway’s brand new Intercity Express Train carried passengers for the first time today, marking the start of the biggest fleet upgrade on the Great Western route in almost 40 years.

“Having already completed 5,000 miles of fault-free running, the first British-built Hitachi train from Paddington to Bristol departed on time at 07.00. It arrived at its destination smoothly and promptly, offering passengers more seats, space and fast wifi. More new trains will follow until the full fleet of 57 are in service next year.

“The Bristol to London service experienced a few technical challenges, for which we apologise to passengers for any inconvenience caused.

“We are of course disappointed with an issue with this train’s air conditioning and water ingress in one of the carriages. We are investigating this as a matter of priority and will restore the train back into passenger service once fully rectified.”