I've remembered a book from when I was little (long time ago!) and even though I can remember a lot of the plot, I can't remember the title or anyone's names and chatgpt has not been able to help - really hoping one of you know it!
I read it in the 1980's but it could easily be earlier as I got lots of hand-me-downs. I'm fairly sure the book was hardback with a cream coloured cover and an illustration of a train station on the front, it wasn't full novel length but wasn't a baby/toddler book.
In the story, some children, 3 of them I think, get on a mysterious train at a train station. It goes through a tunnel and gets to a darker land, that you realise is actually a boy's massive bedroom, the boy in the bedroom is much bigger scale than them and their train forms the toy train of the boy that is in that bedroom.
The boy in the bedroom is ill and they end up going back to their home and getting pills, I presume penicillin, to take back and cure him (they've gone back in time on the train so the boy in the bed doesn't have penicillin). One of the boys had a father who was a doctor or worked in some medical role, so they could get sneak the pills from his house as he received free samples in the post from pharmaceutical companies. They flew the pills to the sick boy on the toy remote control helicopter one of the boys had.
There was a part, when they were going through the tunnel, where they put sweets, like old fashioned hard boiled sweets, I think called 'satins' on the train engine (or whatever it's called) to let them grow bigger when they went through the tunnel so they'd be big enough for the boy to eat. They were also told not to leave the train and there was also a part where one of the train drivers left the train while it was in the large boy's bedroom and froze like a toy person, that's why they were all told not to leave the train.
I can virtually write the book at this point so it's driving me mad I can't remember the name and google it.