Train from Uckfield are starting some journeys a few minutes earlier on weekday mornings.
The new timetable started on Monday and will run until Friday, December 19.
The reason for the change is leaf fall on the line.
A Southern Rail press release said: “Autumn poses a risk to a very small number of our rural routes which can be vulnerable to poor rail conditions due to leaf fall.”
It added that by starting a few minutes earlier any late running caused by slippery rails should be negated, allowing the train to reach its final destination on time.