Busy trains are now being highlighted in online timetables as rail companies urge passengers to stagger journeys.
The morning rush hour has shifted back to between 6am and 7.30am and more trains are being introduced at peak times to allow for more social distancing.
Southern and Thameslink are asking people to wear a mandatory face covering and buy tickets online or with contactless payments.
GTR Chief Operating Officer Steve White said: “From 6 July we will have more peak services and longer trains on our busiest routes so please check before you travel and remember: you must wear a face covering and you should always wash or sanitise your hands before, and after, every journey.
“We are working with business organisations, and other stakeholders, to respond to the easing of lockdown measures but capacity remains limited by social distancing so, where possible, customers should avoid the busiest times of the day which are now between 6am and 7.30am.
“Online journey planners are a great way to check in advance if your train is normally busy.”
One-way and queuing systems are now in place at GTR’s busiest stations, with 19,000 floor signs across the network and 25,000 train signs, posters, adverts and announcements reminding people to socially distance.
A thousand touch-free hand sanitiser points have been put in at stations and an enhanced cleaning regime is focused on touch points such as barriers.
All 2,700 train carriages are sanitised every night and GTR has treated all its trains with a powerful new viruscide which sticks to surfaces, killing viruses for up to 30 days.
This product is also being used to treat stations, offices and other staff locations such as depots.