Police are investigating a "new" crime of cyber-flashing after a commuter received an indecent image on her phone as she travelled to work.
The victim received two pictures of an unknown man's penis on her phone via Apple's Airdrop sharing function.
Lorraine Crighton-Smith, 34, said she felt "violated" and reported it to the British Transport Police (BTP).
Supt Gill Murray said this particular crime was new to her force and urged people to report any other incidents.
Ms Crighton-Smith, who was travelling on a train in south London, told the BBC's Victoria Derbyshire programme: "I had Airdrop switched on because I had been using it previously to send photos to another iPhone user - and a picture appeared on the screen of a man's penis, which I was quite shocked by.
"So, I declined the image, instinctively, and another image appeared, at which [point] I realised someone nearby must be sending them, and that concerned me. I felt violated, it was a very unpleasant thing to have forced upon my screen.`
"I was also worried about who else might have been a recipient, it might have been a child, someone more vulnerable than me.
"My name on Airdrop says Lorraine so they knew they were sending it to a woman. The images were of a sexual nature and it was distressing."
Ms Crighton-Smith called the British Transport Police as she said she was worried about the motives of the perpetrator.
"What's the next stage from sending a naked photograph to a stranger, what happens next, was he getting any sort of gratification from it?"
The BTP has investigated the incident, but said because Ms Crighton-Smith did not "accept" the photograph there was no technological evidence for them to work with and recorded it as intelligence.
Supt Gill Murray said the force had dealt with cases involving Bluetooth but an incident via Airdrop was "new to us".
"Receiving an indecent image from someone you don't know must be very distressing and something we would take very seriously. If it happens to you, our advice would be to remain calm, retain the image and report the matter to police as soon as possible," she said.
"We have a dedicated Cyber Crime Unit who can analyse mobile phones and track data transfers back to suspects' devices. By linking this to physical evidence, such as CCTV footage or witness statements, we can catch offenders and bring them to justice through the courts."
Airdrop is specific to iOS device and Apple Macs. It uses wi-fi and Bluetooth to talk over a short range to other devices, like other iPhones.
Its default setting is for "contacts only", which means only people you know can see you.
But if you want to share your information or your contacts with other people, you may make a change to the settings and change it to "everyone".
"This means that typically in a train carriage, or tube carriage, you can see other devices," commented Ken Munro, a cybersecurity consultant at Pentest Partners.
"That's what's happened in this particular case, someone has enabled everyone and then hasn't then set it back. As a result anyone within wi-fi or Bluetooth range can send something to you that's quite horrible."
He added that Apple could tackle the issue by making Airdrop return to its default setting if it had not been used for 10 minutes or so.
A spokesman for Apple declined to comment.