A popular rush-hour commuter train from London Bridge to Uckfield is to depart three minutes earlier from next week.
The 18:08 service will depart at 18:05.
The change announced by train operator Southern comes into effect on May 17 as part of changes made to Southern’s summer timetable.
First service to be affected will be on Monday, May 18.
The timetabled arrival time at Uckfield remains at 19:21.
Southern said the earlier start would give the service the “best possible chance” of running to time.
Statement from Southern
A spokesman also confirmed slightly earlier starts to some services from Uckfield in the mornings were now part of the timetable.
In a statement, explaining why changes have been made to the timetable, Southern said:
“We run our services on an increasingly congested network.
Growth in railway traffic
“Over the years as more and more people travel into London, train operators have added more and longer trains to the timetable and previous infrastructure upgrades have proved to be insufficient to cope with the growth in railway traffic.
“In addition, the major works at London Bridge that will eventually help to solve these problems are currently putting added pressure on our operation.
“Our timetable is complex and all services are affected by other train on the line.
Disruption can spread quickly
“A delay, for whatever reason, can have an immediate effect on as many as eight more trains.
“It can spread quickly spread, potentially disrupting services at London Bridge, London Victoria and across our network.
“So, we have reworked our timetable overall to increase the reliability of our trains and providing a more punctual service.”