Nothing to do with the end scene of Back To The Future Part 3! I'm trying to remember a novel I read when I was a child, so it was from the 1970s or earlier. It was in English and set in England. It was about a boy who discovered a disused platform at his local railway station, from where he got on a steam train which took him to exactly the same place but in the past. On his first trip he saw himself as a baby in a pram. Later on in the story he was able to travel back to before he was born by "lodging" in the mind of an older man.
The Old Powder Line by Richard Parker
Brian goes for a ride on a mysterious steam train that takes him back into his childhood.
...Richard Parker's The Old Powder Line (1971), a story of journeys from a strange extra platform in a station, where a steam train takes people to their pasts. On one trip teenage Brian meets himself as a baby, and finds his mother half-recognising him.
The time traveller cannot cross the frontier of his age and remain bodily whole, a problem faced when Brian, as a thought-shape, goes back to rescue Arnold from the 1920s and then returns for someone old enough to go back physically. This scientifically arranged chronoclasm is supplemented by astral-plane projection: the comatose body of Arnold, in hospital, shows the very symptoms of Arnold-in-the-past trapped in a tunnel.