Gauge: 1ft 11½in (600mm)
The Criag Ddu Quarry was near Blaenau Ffestiniog and linked to the main line at Tan y Manod by three inclines with an average gradient of 1 in 6 followed by a final much steeper incline down to Tan y Manod on the 1ft 11½in Blaenau and Festiniog line. When this was converted to standard gauge in 1883, the narrow gauge wagons were carried on transporter trucks to Blaenau Festiniog where they could either continue down the Ffestiniog Railway to Minffordd on the GWR or be transhipped to the LMS.
The quarrymen went to work in empty wagons hauled up the inclines but after work, when the inclines had ceased operating, they used gravity cars running on the middle pair of rails between the two tracks of the inclines. As many as two hundred men would descend in this way, each using their own car which was put into a wagon to be taken up again in the morning. The ceir gwyllt was a unique institution made possible by the relatively long inclines with modest gradients.