Hitachi Rail Europe's managing director Karen Boswell said that the achievement was the culmination of nine years of hard work - from creating the £82 million factory at Newton Aycliffe to recruiting a 1,000-strong workforce.
Passenger Craig McCrum, 38, said "water was pouring out" of the air conditioning system but he was "not surprised" at the performance of the train because "GWR is a complete shambles".
However, there was something wearily familiar about the new fleet of high-tech trains that took to the tracks this morning. 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.
She explained that "an air conditioning issue" resulted in water entering the carriage rather than being discharged outside.
But the inaugural service from Bristol Temple Meads to London Paddington arrived 40 minutes late after twice being delayed.
The problems prompted complaints on social media by passengers.
The Department for Transport said: "The new trains are a renewal of the ageing Intercity 125 fleet, which was first introduced in 1976, and they will go on to replace fleets across the country".
Overhead cables for the electric trains have yet to reach Bristol due to budget cuts and delays, so the trains now have to run on diesel from Maidenhead to Temple Meads.
It means the trains, capable of travelling at 148mph, will still have to travel at the speed of the current Intercity trains (125mph maximum) and run on diesel fuel for the last section of the journey from Maidenhead to Bristol.
He said: "These are the smartest trains in the country, probably the best we have ever had in the country".
Hitachi said the longer carriages provided much more space and comfort for passengers.
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