The extra seats means the train operator will have 16,000 extra seats available per day, amounting to 4 million a year.
The operator is also introducing a new timetable to provide post-pandemic travel, as the company says passenger numbers into London are at 56 percent of pre-pandemic numbers.
Scott Brightwell, Operations and Safety Director for Southeastern, said: "The way we all travel has changed post-pandemic, and many of our customers are now using our services differently and at varying times of the day.
"This new and improved timetable delivers a more consistent all-day service and means we’re providing trains, and space, where it’s needed most - which reflects the way people now travel.
"Our customers tell us that reliability and punctuality are their highest priorities. So, we’ve simplified routes to remove bottlenecks which will see more trains running on time, fewer cancellations, and a more reliable service.
"The simpler structure of the timetable, with most trains leaving stations at broadly the same time each hour, means we can more add more trains into the timetable as demand changes.
"As we continue to recover from the pandemic, our focus remains on providing the most convenient and reliable railway for everyone who uses it."
The new timetable will add more services on peak day and morning hours such as Maidstone East to Charing Cross and new peak time services to and from Beckenham Junction to London Blackfriars.
The new timetable will come into force Sunday December 11, 2022.