Originally designed by Webb as a four cylinder compound, in which form the locomotives proved unreliable, they were all rebuilt by Whale as two cylinder engines.
The model takes this a step further, with a single cylinder driving a heavy flywheel, gear driven to the wheels and fitted with slip eccentric valve gear. It runs beautifully on air, the wheels ticking over smoothly at a walking pace courtesy of the reduction drive.
The boiler is spirit-fired, fitted with a six wick burner.
| gauge | 2 1/2 inch |
| length/inches | 17 1/2 +12 |
| width/inches | 4 1/2 |
| height/inches | 7 1/2 |
| weight/kg | 9 |
| wheel material | cast iron |
| axlebox type | plain bearing |
| cylinder material | gunmetal |
| valve type | slide |
| valve gear | slip eccentric |
| boiler type | water tube |
| boiler material | copper |
| safety valve(s) | 1 |
| safety valve type | spring |
| hand pump | 1 |