There is this book but I don't know if it has line drawings.
http://www.michaelsbookshop.com/catalogue/the_ramsgate_tunnels__main_line_public_airraid_shelter___scenic_railway__.htm
There is also this listing from Amazon although I cannot find one for sale.
The Ramsgate Tunnel Railway Unknown Binding – 1968
by Geoffrey Body (Author), Robert Lockwood Eastleigh (Author
Just a thought - The Post office railway under London uses locos built by English Electric which look similar to those at Ramsgate but with no covering bodywork !
I rode on the railway as a teenager and was fascinated by it although it was run down by then.
My Aunt and Uncle lived at Deal so used to visit Kent regularly.
Regards, Colin