Anyone travelling on public transport in England must wear a face covering from Monday 15th June under a new rule.
More than 3,000 extra staff including police officers are being deployed at stations to make sure people comply.
Passengers without a covering will be asked to wear one, or will face being refused onboard or fined £100.
People with certain health conditions, disabled people and children under the age of 11 will be exempt from the rule.
In the coming days, hundreds of thousands of free coverings will be handed out at railway stations. The government says masks can be homemade, such as a scarf or bandana.
As well as on transport, all hospital visitors and outpatients also have to wear masks.
The UK government changed its advice on face masks to stop the spread of coronavirus earlier this month, as more people used public transport to go back to work. It is now in line with the World Health Organization’s advice.
The new rules apply to England and require anyone travelling by bus, coach, train, tram, Tube, ferry or plane to cover their face while on board.
They exclude school transport, taxis and private hire vehicles – although Uber has made face coverings compulsory for passengers and drivers.
The rules apply only while travelling – not while waiting – but the rail industry has asked people to cover their face as they enter a station.