I was lucky enough to travel on that line.Thanks- fascinating. I’ve seen the tunnel exit in the Freshfield industrial park and would love to have seen the railway fully operational
In 1971 there was a day with trains running from Brighton to Kemptown, my father was a train driver and took me on one of the trains, I wasn’t very old but I do remember he knew the driver and we were in the cab with him.
I’ve just found this link about the day.