Enid Blyton 
Five Go Off to Camp 
Famous Five #7

Julian, Dick, George, Anne and Timmy are planning to go camping in a moor with the absent-minded and insect-loving Mr Luffy, a teacher. When they arrive at camp they find that their camping site is close to a farm, and that several old train tracks run under the moors, some of them are unused. They soon make friends with a boy named Jock, who lives at the farm with his mother and stepfather, the farm owners. While exploring the moor, the Five find a railway yard and a tunnel that are apparently abandoned.