Almost £1.2 billion was invested in the railway connection Kent and Sussex to London last year.
Passenger growth across the whole of London and the South East grew by 7.3%.
More than a billion passenger journeys are made on trains to London every year, equating to roughly 3 million passenger journeys each day.
Investments in Kent and Sussex included lengthening platforms, a new station building at Dartford and miles of tracks being renewed.
Dave Ward, Network Rail route managing director said, "More than 4500 trains run every day between London and Kent and Sussex and over the next five years we will be investing more than £2.3 billion in the rail network in those areas."
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Their warning depends on whether negotiations for a post-Brexit Britain do not offer freedom of movement for people, capital and goods.
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