The first trains are to run through a new £26m railway station in Rochester ahead of Monday morning's rush hour.
The new station, which is closer to the town centre, allows longer 12-car trains to pass through the station. The old station used to take 10-car trains.
Five extra trains to London will run in the mornings and one extra return train will run in the evening.
Network Rail said passenger numbers at Rochester had doubled over the past decade to 1.3m journeys a year.
Route managing director Alasdair Coates said the upgrade would increase capacity and reliability and offer better local transport links.
Southeastern managing director David Statham said Medway's commuter network was growing and commuters would benefit.