There were almost 1,500 such offences reported to British Transport Police (BTP) in 2016-17, the BBC said.
This figure is an increase from 650 reports in 2012-13, a Freedom of Information request to BTP from Radio 5 Live showed.
The broadcaster said most of the reports, which cover England, Scotland and Wales and include the London Underground, were in relation to sex assaults on females aged 13 and above.
BTP Detective Chief Inspector Darren Malpas said an awareness campaign launched two years ago by the force is likely to have prompted an increase in reporting.
He told the BBC: “When the ‘Report it to stop it’ campaign launched, we fully expected to record a rise in sexual offences and it is pleasing that previously reluctant victims of sexual offences now have the confidence to report this to us.
“Tackling all forms of unwanted sexual behaviour on public transport is a priority for British Transport Police and we have worked hard in recent years to send a clear message to victims that they will be taken seriously and we will investigate offences.”
The campaign encourages people to report “anything of a sexual nature, including rubbing, groping, masturbation, leering, sexual comments, indecent acts, or someone taking photos of you without your consent”.