The metaphysics of time travel: Richard Parker, The Old Powder Line. Thomas Nelson Inc., Camden, New York, 1971.
A very Geoffrey Trease feel to this book which may be as much about period as about style (although I would never have guessed it was American).
Brian wanders to the old station one day and ends up on a steam train which takes him to watch his own past. He tells his father's secretary, Miss Mincing who tells her brother Arnold. Arnold is in his forties and in a wheelchair. One of the nice things about this book is that it presents friendship between a fifteen year old and a forty year old plausibly and without sentimentality. Arnold gets stranded in the past and Brian, with the help of his sister's friend Wendy, goes back to try and rescue him/persuade him to return.
The time travel is sort of metaphysical, but there is a lot of explanation that works very well on p. 103. Also endearing is that Wendy, who gave up science in the third year, follows it all.
"I did, but you don't have to hand in your head with your physics and chemistry books."
Brian hadn't come across any really intelligent girls before. He was a little bewildered and would have liked to explore further. (p. 104)
Such is young geek love, 1971 style.