Southeastern has started operating its 200 additional daily trains for the Olympic Games.
The company is running a special timetable until 12 August and between 29 August to 9 September.
It includes trains starting earlier and ending later on some routes and extra carriages being added to services.
The rail firm added that on Monday London Bridge platforms 1-6 would be exit only between 18:00 BST and 22:00 BST.
The company said Monday would be the busiest day, with 50,000 people expected to attend the equestrian cross-country event at Greenwich Park.
The company's high speed service has become a shuttle service between St Pancras, Stratford, and Ebbsfleet, with the occasional train going onto Faversham.
Southeastern also has 1,000 additional temporary staff to assist passengers over the summer.
Alison Nolan, from the firm, said: “From this weekend, our services will be extremely busy.
“Queues are to be expected at many London terminal stations.
“We recommend passengers travelling during Games-time check to see how their journey is likely to be affected.”
The rail firm added that only essential maintenance on its network would be carried out during the Games.