The first journey of the new Inter City Express service from Bristol was marred today by delays, a breakdown, and water entering the carriage.
Hitachi Rail Europe managing director Karen Boswell said she was "very sorry" that the train had been hit by technical issues.
The Great Western Railway service, which counted transport minister Chris Grayling among its first passengers, departed 25 minutes late from Bristol Temple Meads this morning. It then came to an unscheduled halt at Taplow station in Berkshire while engineers worked on a technical fault.
Boswell explained that a problem with the air conditioning system had resulted in water pouring into the carriage.
"This was not to a standard that Hitachi expects and is known for," she said "We can and will do better. Our depot teams are as a matter of priority investigating the root causes of today’s technical issues, and we will ensure that these are corrected as quickly as possible."
Several other trains were also delayed as the Inter City Express blocked the high speed line, slowing down the journeys of thousands of commuters.
It eventually arrived at London Paddington about 40 minutes later than the planned time.
Five other journeys were completed on the new service today, all without incident.
Passengers complained on social media that several people were forced to stand on the journey, despite GWR's pledge to increase capacity.
Read more: Heathrow expansion is ready for take-off
Earlier in the day, transport secretary Chris Grayling praised the new fleet, which he said would give passengers "faster, more comfortable trains and better journeys".
"This is part of a record £40bn investment to transform our railway and deliver better journeys for passengers," he added.