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 diesel Moelwyn and three 4-wheeled carriages. This was often called the Dduallt Diddy or sometimes just the Diddy.
It originally consisted of Moelwyn, quarrymen’s carriage 8, bug boxes 5 and 6, and brake van 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.
During the week the train would normally be stabled in the bottom siding at Tan y Bwlch, being returned to Porthmadog at weekends for servicing. As the bug boxes didn't have vacuum brake cylinders the guard would apply the hand-brake in the brake van while the locomotive ran round at both Tan y Bwlch and Dduallt.
In 1972 the bug boxes were replaced by new open 'Toastrack' Carriage 37. As this vehicle at the time had a hand brake wheel on the bottom bogie, the brake van was turned round so the guard could use the more reliable brake on 37, being reached from the balcony on van 2.
Operating the Dduallt Diddy could be quite fun for the train crew, dashing up and down the line between the main service trains. No lights were provided so passage of Garnedd Tunnel would be in the dark, with suitable sounding of Moelwyn's klaxon horn to frighten a group of Girl Guides on at least one occasion. It was also not unknown for the fireman (secondman?) and guard to swap places.
Its purpose was to allow passengers arriving at Tan y Bwlch to enjoy at least a short ride without having to be crammed into an already busy Porthmadog to Dduallt service. It certainly served its purpose for several years, even if it was a bit rudimentary by the standards of the main trains of the day.
Ffestiniog Railway Magazine, Issue 050 (Autumn 1970)