Rail minister Theresa Villiers has launched the first set of new 12-carriage trains to run on a busy commuter route.
The MP rode in the driver's cab on a First Capital Connect (FCC) train from St Pancras to Farringdon station in the City of London.
"These longer carriages will make a real difference to commuters travelling on these busy routes," said Villiers.
"The Thameslink project is a key part of our massive programme of rail improvements."
Although her train was on time, another Bedford-bound FCC train was 11 minutes late during the rush-hour - due to the late arrival of a crew member.
The 12-carriage trains were able to run on the Bedford to Brighton line due to the extension of platforms as part of the £6 billion Thameslink project which will be completed in 2018.
Passengers, including Villiers, who arrived at Farringdon today were able to sample the new ticket hall, part of a massive transformation of the station which also serves the London Underground.
The longer trains are providing FCC passengers with an extra 3,200 seats a day.
Villiers was asked about a report that the commercial director of the East Coast train company - run in the public sector - took seven plane journeys last year between London and Scotland even though he could have used his own company's trains on the route.
"I am not going to comment on personal travel arrangements," said Villiers.
"East Coast has put in a huge effort on this line and has been hit by problems, including cable theft. It's important that East Coast performance improves."
As passengers tried the longer trains, some other travellers had less successful journeys today.
A signal failure at Twickenham in west London led to delays for passengers on South West Trains' services in and out of Waterloo station.
Buses replaced trains between Bedford and Bletchley due to a broken-down train at Kempston Hardwick in Bedfordshire. Passengers with the London Midland train company were affected.
In Kent, a signalling problem at Sittingbourne led to delays to Southeastern train services between Sittingbourne and Sheerness-on-Sea.
Another signalling problem - at Guiseley in West Yorkshire - caused delays to Northern Rail services between Bradford Forster Square/Ilkley and Leeds.
On London Underground, there were morning rush-hour delays to District line services due to the late finish of engineering work at Earl's Court in west London.