Britain's train companies will ramp up services this week as the Government eases coronavirus travel restrictions.
Industry body the Rail Delivery Group said services will increase from 50 per cent of the standard timetable to 70 per cent from Monday.
However, the capacity on each service will be reduced to as little as 10 per cent of normal levels in a bid to enable social distancing.
Meanwhile, passengers are still being urged to avoid public transport where possible as well as non-essential travel in general.
British Transport Police will deploy more officers at London stations in a bid to control crowds.
Will Rogers, managing director at East Midlands Railway, warned that the new timetable "will only allow a small rise in the number of passengers we can accommodate".
He added: "We urge everyone to only go by train if it is necessary and keep public transport for key workers and those who must travel."
Passengers travelling by train are being asked to wear a face covering and keep a two metre distance from other people where possible.
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.
Passengers using London North Eastern Railway are only allowed to board trains if they hold a reservation as well as a ticket.
The operator is asking passengers to sit in a window seat, with one person per row of four seats, and two empty rows between each passenger.
People travelling as a household will be allowed to sit together but must maintain "a safe distance" from other passengers.
Avanti West Coast warned its customers that anyone without a reservation may not be able to travel on their choice of train due to capacities being limited to around a quarter of normal levels.
Train operator Northern said there will be "significantly reduced capacity on each and every one of our trains".
Rail services have been slashed for weeks due to the coronavirus pandemic causing a collapse in demand and a rise in staff sickness.
But the Government is now urging people in England to go to work if they cannot work from home.
Advice in Northern Ireland, Scotland and Wales remains that people should stay at home.
Meanwhile the Rail, Maritime and Transport union described the increase in train services as a "high-risk strategy" and expressed concern that "rushed political considerations could well override the safety issues for staff and passengers".
It has called for new compulsory protections for passengers and rail workers, including the enforcement of two-metre social distancing on trains and the compulsory wearing of face masks by passengers, which should be provided for free at stations and be able to be disposed of safely.
RMT general secretary Mick Cash said: "We are opposed to the early relaxation of lockdown measures and believe that non-essential workers should avoid using trains. When people absolutely must use a train, there should be new compulsory protections.
"We have the crazy situation of Eurostar passengers arriving with masks on into St Pancras but then not wearing masks when they transfer to the tube or other rail services."
A Department for Transport spokesman said the message remained that people should only go to work if they cannot work from home and they should avoid public transport if possible and maintain social distancing if they have no other choice.
He added: "We have asked operators to increase the number of services from today to help reduce pressure on the transport network, providing more space for social distancing as well as delivering increased reliability and extra capacity for the future."
