 Ruislip Lido is a nineteenth century reservoir with an artificial sand beach , surrounded by woodlands through which runs the Ruislip Lido Railway , a miniature 12 " gauge railway with diesel and steam locomotives . The Lido was established as a reservoir to feed the Grand Union Canal by damming and flooding the lower part of the valley between Park Wood and Copse Wood , including the hamlet of Park Hearn . Work began in 1811 ; the consulting engineer John Rennie announced completion of the project to the Grand Junction Waterworks Company on 5 December that year . Unsuccessful as a canal feeder lake , it was reconstructed as a pleasure resort in 1933 . 
