When the line was extended to Dduallt in 1968 there were quite a number of potential passengers who arrived at Tan y Bwlch. Often the trains arriving there were already full and the solution was to run a Shuttle to Dduallt consisting usually of Moelwyn and the 4-wheeled carriages. This was often called the Dduallt Diddy or sometimes just the Diddy.
FRM-050, Autumn 1970 contained quite a detailed description of the "Diddy". It consisted of Moelwyn, quarrymen’s coach No.8, bug boxes Nos.5 and 6 and brake van No.2. It ran on Mondays to Thursdays as directed by Control, leaving Tan y Bwlch immediately a main train arrived at Dduallt and running into the loop at Dduallt when the main engine had run round its train. The main train would then leave three minutes earlier than shown on the public timetable, to be followed by the shuttle on clearing the section. By the time the shuttle ran into Tan y Bwlch the down main train had normally vacated the down platform and the up train had taken water and was ready to leave for Dduallt. The shuttle engine would then have ten minutes for running round and servicing, before leaving for Dduallt again.