Trains were still packed on Monday morning as commuters failed to listen to the government's coronavirus advice to keep two metres away from each other.
Pictures from several routes into London and the Tube showed carriages as full as usual, with little to no space in between commuters.
Train services across the UK have been reduced, as of Monday, after customer demand for rail travel fell by up to 69% on some routes since Boris Johnson issued advice last week to stop all non-essential journeys to help prevent the spread of COVID-19.
London Underground, Overground, DLR, TFL Rail and London Trams services are also operating a reduced service from Monday.
The government said key services would still run to ensure people could get to work, travel to medical appointments and goods can be transported.
However, many of those commuting on Monday morning said the number of carriages had been reduced and timetables cut, resulting in overcrowding.
The 6.30am C2C service from Grays to Fenchurch Street on Monday was particularly full, with commuters left with no option but to squeeze up together.
Some travellers did report more empty trains than usual, with the South Western Railway service at Clapham Junction "quiet like a Sunday", one traveller said.
The UK is facing the prospect of a total lockdown after people over the weekend failed to listen to advice to stay two metres apart, with thousands visiting British holiday hotspots including Snowdonia, Cornwall and the Scottish Highlands, as well as city parks.
As well as train timetables being reduced, the government has brought in emergency measures to provide refunds to advance and season tickets holders.
The Department for Transport also announced it is suspending normal rail franchise agreements and will take over all the risks, as well as revenues, for operating trains for at least six months to help train companies cope with a significant drop in income.
On Sunday afternoon, Boris Johnson said more extreme measures will be considered over the next day to prevent coronavirus spreading if people did not stay two metres apart.
Health Secretary Matt Hancock told Sky News' Kay Burley@Breakfast on Monday: "We will enforce and bring in further strong measures if we need to - but I'd much rather people follow the rules themselves, it would be much more straight forward."
"If we need to go further in terms of people's interactions then we will."
MPs are returning to the Commons after the weekend to debate the government's Coronavirus Bill in all its stages, before it goes to the House of Lords and becomes law by the end of this week.
The bill, which runs to more than 320 pages, would allow police to have powers to detain suspected carriers for quarantine and screening, restrictions on travel, streamline NHS procedures and close ports and airports if there is a shortage of Border Force staff.