The first Intercity Express Train (IET) to run a passenger service caused rush-hour mayhem this morning when it came to an unscheduled halt at Taplow station in Buckinghamshire, with Transport Secretary Chris Grayling and other dignitaries on board.

The 6am from Bristol Temple Meads to London Paddington blocked the fast line, delaying thousands of passengers aboard express trains stuck behind it as engineers worked on what was described as a “minor technical fault”.

The Japanese-built Hitachi train had a mix of commuters, train enthusiasts, rail executives and politicians on board. It had left Bristol 25 minutes late due to a separate fault.

Shortly after leaving Reading, with an estimated 1,000 passengers on board, the IET was due to switch from diesel to electric overhead wires at Maidenhead.

But something went wrong and it came to a grinding halt 22 miles from its final destination.

At peak times, fast trains are scheduled to pass through Taplow at 125mph every few minutes. Some were diverted to the slow line, but two high-speed trains – including one which had been held at Reading station to allow the flagship train to proceed – were unable to move until the obstruction was cleared.

One service from Bristol Temple Meads via Newbury was delayed by almost an hour.

The maiden journey for the train eventually arrived more than 40 minutes late. It had taken almost two-and-a-half hours since the booked departure for the 120-mile journey, at an average speed of only 50mph. Passengers were entitled to claim compensation for the delay.

The same rolling stock was due to leave Paddington at 8.15am for Cardiff, but the service was cancelled – to the disappointment of rail enthusiasts who had planned to travel on the first IET to South Wales.

Karen Boswell, managing director of Hitachi Rail, later apologised to passengers in a video statement, saying: “Today marked an important milestone for Hitachi and the partners in the Inter City Express programme, as the first new passenger services were brought to the Great Western route.

“I am, however, very sorry and disappointed that today’s first passenger train from Bristol encountered technical issues, causing a delay to the service and an air-conditioning issue which resulted in water entering the carriage rather than being discharged externally. This was not to a standard that Hitachi expects and is known for.