Train companies are urging people to avoid non-essential travel after recording an increase in journey planning of nearly a quarter this week.
Rail services will be ramped up from around 50% of the normal timetable to 70% from Monday as part of the easing of coronavirus lockdown restrictions.
But there are concerns that social distancing will not be possible if demand for travel is too high.
Industry body the Rail Delivery Group (RDG) said 3.7 million journeys were planned between Sunday and Thursday, compared with three million over the same period one week earlier.
To keep people safe, there will only be space on trains for as few as a tenth of the usual number of peopleRobert Nisbet, Rail Delivery Group
Passengers are being urged to drive, cycle or walk to their destinations instead of using public transport.
The RDG warned that the capacity of trains will be restricted to as little as 10% of the normal number of people who travel.
Those who have no alternative means of making their journey are asked to avoid peak times.
People embarking on long-distance journeys are “strongly advised” to reserve a seat otherwise they may not be allowed to travel.
RDG director of nations and regions Robert Nisbet said: “To keep people safe, there will only be space on trains for as few as a tenth of the usual number of people despite train companies gradually increasing services next week.
“We need people’s help to keep trains clear for those who really need them, so we are asking people to consider alternatives like cycling or walking, and if that’s not possible, to travel at quieter times.
“We are doing everything possible to keep passengers and staff safe, including cleaning trains and stations several times a day, managing capacity and increasing signage to help people get around.
Only use public transport if you cannot work from home, or cycle, walk or drive. Stay alert, control the virus, save lives.— Southern (@SouthernRailUK) May 13, 2020
Find out more 👉 https://t.co/vCwKncpF0n #TravelSafely #StayAlert pic.twitter.com/Ifj50IodNH
“For those who do need to take the train, we ask that they plan ahead, consider others and stay safe when they use public transport.
“That means buying tickets online for quieter times of day, maintaining social distancing wherever possible, wearing a face covering and keeping hands sanitised.”
Guidance issued by the Department for Transport on how to travel safely during the coronavirus outbreak states that passengers should minimise the time they spend near other people and avoid physical contact with them.
It advises them to face away from each other when they cannot keep a two-metre gap.
Transport operators are being urged by the Government to rearrange, remove or limit seating “to try and ensure social distancing is observed”.
This may include blocking off seats in close proximity to others and removing face-to-face seating.
The RMT union claimed images of packed London Underground trains on Wednesday showed how “fraught with danger” the Government’s return to work message is.