There was an increased police presence at train stations on Monday morning as rail companies ramped up their services amid the coronavirus lockdown.
Network Rail warned it will close stations if there are too many commuters and safe social distancing isn’t possible.
As well as extra police, security guards trained in crowd control have been put on duty at major railway stations in England, where more frequent trains have started running.
The increase in services follows the easing of lockdown restrictions in England last week by Boris Johnson.
The Rail Delivery Group said services this week will increase from about 50% of the standard timetable to 70%.
However, capacity will be reduced to as little as 10% of normal levels to ensure social distancing, while passengers are being urged to avoid non-essential travel.
In London, there were more British Transport Police officers at stations on Monday morning.
Passengers have been asked to wear face coverings and keep two metres apart from others.
Transport operators have been 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.
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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.
Network Rail, which manages Britain's 20 busiest stations, said passenger numbers were "very similar" to last week, when they were around 93% below average.
Its chief executive Sir Peter Hendy told BBC Breakfast on Monday: "We have processes to close stations if they become too full.
"My railway colleagues have rehearsed those processes on the national railway network if that becomes necessary."
He said face masks are not mandatory for passengers but "we are all very keen in the rail industry" for people to wear them.
"We are relying on people to be sensible. We are doing our best too,” he said.
"We want people to stay apart if they humanly can and if they can't, then a face covering is a quite sensible thing to do for the brief moments when you might be getting on or off a train or moving through a station."
He said a number of stations have been given one-way systems and a "huge" amount of work has been done on signage.
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On Monday, extra police officers and security personnel were deployed to deal with any overcrowding, but many major stations such as London Euston, Birmingham New Street and Reading were quiet during the morning peak.
Staff at Clapham Junction in south-west London said its crowd control barriers were not required as there was no noticeable increase in travel.
Commuter Daniel Croft, 37, who travels twice a week from his home in Darlington to London King's Cross, said: "The trains have been completely empty."
Victor Stringer, 69, who was returning to his home in Peterborough after finishing work in the capital, commented: "It's been so quiet, I could almost have realised my boyhood dream of riding up front with the driver."
King's Cross station, one of London's busiest transport hubs, was almost deserted on Monday morning.
Its concourse was dotted with stickers reading "Protect your NHS, stay 2m apart", while regular announcements urged people to stick to social distancing measures.
There were many more staff on duty than travellers, as well as several police officers on patrol around the station and neighbouring St Pancras.
The RMT 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 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.
A Department for Transport spokesman said: "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."