New measures have been deployed on trains and at stations amid fears that more people might use public transport to return to work in England this week.
Train firms have operated reduced services due to coronavirus, but more frequent trains are now running.
Security guards with crowd management training will be at some stations.
People are being encouraged to go back to work in England, but to only use public transport for essential journeys when they have no alternative.
Some rail bosses have expressed concerns that social distancing will not be possible on trains or platforms.
Reservation only
And more radical steps, such as requiring passengers to book time slots for when they can arrive at a station, are being considered.
Train operators are already planning to limit numbers boarding specific services. Passengers could be prevented from boarding a train or entering a platform if there are already too many people.
Many intercity trains will be reservation only and Avanti West Coast has said it would not allow carriages to be more than a third full.
Some train companies will block off seats to ensure that passengers spread out. It is also possible that if a service becomes relatively busy early on, then the train will not stop at other destinations along its route.
In future, train operators might not open the doors of certain carriages at earlier stations along a route so that people can get on at a later stop and still have the necessary space to keep their distance.
One-way systems are already in place at stations, such as Clapham Junction in south-west London, to ensure that social distancing is possible.
The chairman of Network Rail, Sir Peter Hendy, was asked by the government to create a plan for managing the flow of passengers.
Stations have been reorganised, signs have been installed and space could be made outside for queuing in case entrances and exits are closed.
Sir Peter, who is one of the most respected and experienced figures in the railways, said he was "confident" that enough work had been done prior to Monday's increase in services, so that stations "were in a better place".
Currently, only people who cannot work from home are advised to return to their workplace.
Employers had a vital role to play, said Sir Peter, because they can "strongly influence people's behaviour".
"You wouldn't set out for work if someone told you not to come," he said.
People 'fed up'
PC Jason Kelly said the number of passengers on his train from King's Cross to north Hertfordshire had risen from two to up to 40 after lockdown measures were eased last week.
The officer, who was returning home after a night shift, was not confident that social distancing measures could be met if passenger numbers rise further.
"For some people it's just like a normal day, people have got fed up with [coronavirus], they've had enough," he said.
Transport for London has spoken to around 300 businesses and the Transport Secretary Grant Shapps has indicated that office start times will be staggered to manage demand on public transport. Meanwhile, London's congestion charge, which was suspended in March, will be reintroduced on Monday.
The central message from the government is that people should avoid public transport if they can.
People are also being encouraged to cover their face when on board a train or at a station when it is crowded.
However, senior figures from the rail industry insist they will not be policing whether people are following government guidelines. One rail source said the industry had "done everything we can to suppress demand".
Passenger numbers last week were slightly up on the previous week. However, footfall through major UK stations was only about 10% of pre-pandemic levels.
During the crisis the government is covering the losses made by train companies, which saw revenues evaporate when travel restrictions began.
But industry forecasting predicts that significant passenger numbers will return several weeks from now.
Train companies admit they are nervous about how the situation can be managed, once crowds return. "We are counting on the individual conduct of passengers", said one rail source.
"When we say essential journeys only we really mean it."
Mick Cash, the general secretary of transport union RMT, said: "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."
Source: BBC