The first three Thameslink trains from Brighton each morning will set off early to try to overcome the problem of leaves on the line.
Train operator First Capital Connect said that the 5.10am, 5.40am and 5.50am services would all leave up to three minutes early.
The timetable changes will affect weekday services from next Monday (8 October) until Friday 7 December.
The company said: “The timetable adjustment is made every year to combat the effect of slippery autumn leaves.
“Details can be found on the First Capital Connect website and on posters and leaflets at affected stations.
“Autumn leaves are a serious problem that affects railways throughout northern Europe and North America.
“Fallen leaves land on the rail surface and are crushed by the train wheels to form a thin but extremely slippery coating on the rail similar to black ice on roads.
“This is not dangerous but it reduces the rate at which trains can speed up and slow down, which in turn extends journey times.”
From Monday the 5.10am from Brighton will set off at 5.07am, the 5.40am will leave at 5.37am and the 5.50am will leave at 5.48am.
The 5.14am from Preston Park will set off at 5.11am, the 5.44am will leave at 5.41am and the 5.54am will leave at 5.52am.
Meanwhile, Network Rail is cutting back line-side vegetation and running special trains to remove leaf mulch from the rails.
First Capital Connect said: “Extending a couple of morning journey times by a few minutes will allow us to deliver a more consistent service through the autumn.”
