Bombardier announced 1, 400 job cuts after losing out on the Thameslink deal in June.
The first 12-carriage trains have been rolled out on the Thameslink route between Bedford and Brighton.
BBC: Longer trains set to ease congestion on Thameslink line
Thameslink, a major commuter line between Bedford and Brighton, has a station opposite King's Cross.
Other franchising competitions, including for Essex Thameside, Great Western and Thameslink, are currently suspended.
Four train companies - Thameslink, Thames, Silverlink and WAGN - are operating to emergency timetables.
The Thameslink scheme was first proposed in 1991, when it was optimistically known as Thameslink 2000.
The Department for Transport awarded the contract for 1, 200 new carriages for the Thameslink line to Siemens.
I've also received a few complaints about the new bridge at Blackfriars built as part of the Thameslink redevelopment.
The union is seeking to overturn the Department for Transport's decision to award the Thameslink deal to Siemens over Derby-based Bombardier.
The contract will create 1, 400 jobs in the UK. Siemens is to build two new depots to maintain the Thameslink trains.
BBC: Siemens let off steam in latest Bombardier rail jobs row
The company is part of a consortium which will build and maintain 1, 200 vehicles and two depots for the massive Thameslink project.
The company's other service, Thameslink, has also been affected by the work-to-rule.
The Thameslink upgrade means trains running from Bedford and Brighton will use a 12-carriage service instead of eight and will eventually lead to a more frequent service.
Longer trains will be able to call at Radlett, Elstree and Borehamwood, Mill Hill Broadway and West Hampstead from 2018 when the final Thameslink Programme timetable is introduced.
BBC: Longer trains set to ease congestion on Thameslink line
The trade mission also offered the chance for some informal lobbying over Bombardier's loss of a multi-million Department for Transport contract to build new carriages for the UK's Thameslink.
There are four franchises due to be decided next year - Greater Western, East Coast, Thameslink and Essex Thameside - and about 15 due before the next general election.
Securing the Thameslink contract was regarded as crucial for the Derby site - the UK's last remaining train factory - as most of its current orders will soon be finished.
BBC: Derby leaders plan to visit Bombardier bosses in Berlin
Southeastern trains said it would be operating a Saturday timetable service on mainline, metro and high-speed routes on Tuesday, with a normal weekday service for Thameslink trains run via Blackfriars.
Philip Hammond told a committee of MPs on Wednesday that 1, 400 redundancies at the firm's Derby factory were "regrettable" and added that delaying the Thameslink trains contract would be unacceptable.
BBC: Miliband calls for Commons debate on Bombardier layoffs
Train company First Capital Connect said it was running a normal timetable but spokesman Roger Perkins predicted delays on Thameslink through Bedford and the Great Northern route to King's Lynn in Norfolk.
There is also the possibility that it will open the door to claims from those companies affected by long delays to two other big franchises, Essex Thameside and Thameslink, Southern and Great Northern.
It explains why the department has suspended the competitions to run the Great Western, Essex Thameside and Thameslink services, just in case the financial models underpinning those contests are flawed in a similar way.
So with the franchise process suspended, affecting the West Coast route, three franchises including Great Western and Thameslink that were already on track, and throwing into doubt future franchising throughout Britain, we've come to a situation that would stretch the imagination of the writers of BBC2's outrageous satire The Thick Of It.
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