Initially, the design incorporated a rigid 'translated' trailing axle with outside plate springs. The first locomotive No. 1009, was fitted with this feature, but it does not appear to have been very successful. No. 1010 may have also been initially fitted with the translated axle, but all of the other C12s were built with a more conventional radial truck design. It is thought that No. 1009 was converted to use the radial truck design, but no official records exist to confirm this.
The C12's boiler was of the same standard Ivatt domed type which was used on the D4, E1,J4, and J50 classes. The working pressure varied over time between 170psi and 175psi.
The fifty C12s built for London services were initially fitted with condensing gear for operating the Metropolitan Widened Lines. The first locomotives had a condenser pipe leaving the smokebox just above the boiler handrails and continuing to the front of the water tank in a continuous line. This was brass with a cast iron elbow into the tank. Later locomotives had the entire pipe made of cast iron, and extended down to the bottom of the smokebox (see picture). This arrangement became the standard for all condensers fitted to Great Northern (GN) locomotives.
|Motion:||Stephenson (slide valves)|
|Boiler:||Max. Diameter:||4ft 5in|
|Heating Surface:||Total:||1119 sq.ft.|
|Tubes:||1016 sq.ft. (213x 1.75in)|
|Grate Area:||16.25 sq.ft.|
|Tractive Effort:||(@ 85%)||17,900lb|
|Total Wheelbase:||27ft 3in|
|Engine Weight:||(full)||62 tons 6cwt|
|Max. Axle Load:||18 tons|
|Coal Capacity:||2 tons 5cwt|
|Water Capacity:||1350 gallons|