The weekday timetable of Thameslink trains will change from the first Monday of September, with more services being introduced.
Govia Thameslink Railway says the new timetable is designed to meet the needs of customers across the network and support schools, colleges and those returning to the office.
Almost 1,000 trains per week will be reinstated when the new timetable comes into force from September 6.
On the line into and out of London St Pancras, there will be an increase in services on weekdays.
For example, for passengers using Elstree & Borehamwood and Radlett stations, the number of off-peak trains will rise from four every hour to six.
During peak hours, six trains are already running although some of the timings may change when the new timetable comes in.
Govia Thameslink Railway chief operating officer Steve White said: "In anticipation of the return of schools and more passengers returning to the office, much of our service will be expanded from September 6.
"Like many other businesses our staffing levels continue to be affected by the pandemic but the new timetable has been planned to make better use of the drivers we have available, providing more trains, more reliably."
Mr White added: "The Covid third wave this summer has been the most challenging throughout the pandemic to balance the availability of our resources with the needs of our customers. We apologise if you have been affected.
"These changes will ensure we provide you with an increase in trains and provide a more reliable service as we steadily work to overcome the legacy of the pandemic. As people come back to using the railway, we will continue to adapt our services to match demand."
Train times at www.nationalrail.co.uk will be updated from the weekend of 28 August, from when passengers are urged to check and plan ahead.
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