Class S (55) 4-6-2
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| Origin | FMSR | Builder | North British Locomotive Co (Scotland), Beyer Peacock (England) |
| FMSR Class | S | Cylinder | 17 x 24 inches |
| FMSR Number range | 237-252 | Boiler Pressure | 180 psi |
| KTM Class | 55 | Driving Wheel Diameter | 54 inches |
| KTM Number range | 551.01-551.03, 552.01-552.08, 553.01-553.05 | Water Capacity | 3,000 gallons |
| Introduced | 1928 | Weight Full | 108 tons |
| Extinct | 1959 | Tractive Effort | 29,480 lb |
Class "S" was delivered in three batches between 1928 and 1930. The first two batches were from North British, and the third from Beyer Peacock. The S class was a big step forward, having 3 cylinders and consequently more power than their predecessors. The earlier batches had Gresley conjugated valve gear, whilst later examples had Rotary Cam valve gear. At some stage, these locos were modified with smoke deflectors and sloping running plate, giving a much more modern appearance, further enhanced by conversion to oil firing after the war.