For the next three months, passengers travelling on Southern, Thameslink, Gatwick Express and Great Northern services will see an appeal to help find a missing person when they log onto train wi-fi.
One of the missing people featured in the appeal is Georgina Garsallah, a mother-of-two who was last seen in her home town of Worthing, West Sussex, on 7 March 2018.
Her mother Andrea Gharsallah told BBC Radio Sussex: “Somebody must have seen her that day.
"Somebody might know something, and that’s all we hope.”
Laura Campbell, safeguarding and wellbeing manager for Govia Thameslink Railway (GTR), said: “Over the past year, we’ve seen a concerning increase in reports about vulnerable people on our network.
"We know that winter can be a tough time for many, so it’s really important we’re looking out for those most at risk."
Paul Joseph, head of the helpline for Missing People, said members of the public "are really vital” in solving long-term cases like Ms Gharsallah's.
“You never know what might trigger a memory or a little bit of information from five years ago,” he added.
Andrea Gharsallah said: "“Over the years we’ve always lived with hope, especially as the police have got no evidence at all as to what happened to Georgina.
"Anything is a possibility.”
Mr Joseph said: "As the clocks go back and the days become darker, there’s even more concern for people who are missing or at risk of going missing."