Anyone travelling on public transport in England must wear a face covering from Monday 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.