Disruption to the Thameslink service, due to work on a new station at St Pancras, comes to an end on Monday.
Work on the new station began in September
The eight months of work meant trains from the north ended at St Pancras, while southern services terminated at King's Cross or Blackfriars.
The Channel Tunnel Rail Link (CTRL) has now competed work on what will replace King's Cross Thameslink station.
But passenger groups criticised an interim timetable, to run to 12 June, which halves off-peak services.
According to the Rail Passengers Committee (RPC) and the London Transport Users' Committee (LTUC), the interim timetable will be introduced to allow for train driver re-familiarisation with the route prior to them driving trains in passenger service.
Brian Cooke, chairman of LTUC, said: "Thameslink customers have suffered for eight months, and return to a less frequent service than before, simply because the rail companies cannot get their planning joined up."
The construction work was carried out to compete a "box" under St Pancras station.
It will ultimately replace King's Cross Thameslink station on Pentonville Road in north London.
According to CTRL, it is large enough to accommodate a twin-platform station that will allow passengers direct access to the refurbished St Pancras International which is scheduled to open in 2007.
Tim Smart of CTRL said: "The on time re-opening of the Thameslink through route will mark the achievement of another important milestone for the CTRL and is the result of a tremendous effort by our contractors and project management team."
Thameslink trains run from Brighton and Sutton to Bedford.