M Class De Glehn Pacifics
Click here for photo of the M Class.
Click here for photo of the M Class.
Click here for photo of the M Class.
In the India of the early-1920s, steam locomotive technology was outdated. The engines were almost always of prewar British design. Their narrow fireboxes could not burn high-ash Indian coal well enough due to restrictions in air supply and ash disposal problems. Inside cylinders were still prevalent and "hot boxes" were common because of poor bearing lubrication. The newly-formed Indian Railways Standard (IRS) Committee decided to introduce a new range of engines with wide fireboxes. One of them was the XC 4-6-2.
But the Bengal Nagpur Railway (BNR) always tried to be different from other Indian railways. They had had good experience with the K Class Atlantics which were 4 cylinder de Glehn compounds. Therefore, in 1929, they ordered 18 Pacifics which were based on the XC class engines and had the same chassis and boiler design. However, these were to have 4 cylinders operating as per the de Glehn compounding system.
TECHNICAL NOTES
The M Class 4-6-2 was, perhaps, the most advanced steam locomotive to ever run in India. In fact, on paper, at least, it was, arguably, the most advanced steam locomotive to be in service in 1929. That is a tribute to the management of the BNR, who wanted such an engine to run on their lines. The M Class also had the highest tractive effort of any express loco to serve in India.
It could work as a simple expansion engine during starting and developed an enormous tractive effort of 37 000 lb due to its high boiler pressure of 250 psi made possible by the good quality coal available close to the lines of the BNR. My father once told me that he had seen the impressive hauling power of an M Class at the head of an express as it pulled away from a station faster than trains hauled by lesser locos. His observation accords well with the fact that these Pacifics could maintain a speed of 45 mph while hauling a 420 ton train up a 1 in 100 gradient.
While, on paper, the M Class was a technically advanced machine, there is no evidence that it was any more efficient than a regular XC (on which they were based) because de Glehn compounds built before Andre Chapelon's researches in France did not have the oversized steam passages and good draughting which made the later French de Glehns arguably the finest steam locomotives ever built. The XCs themselves consumed 10 % more coal than did their successors, the far superior WP streamlined Pacifics. Unlike the Vulcan-built XCs, the M Class engines were supplied by the North British Locomotive Company. The M Class too was, reputedly, a poor steamer. Though they may not have met all expectations, these 4-6-2s reportedly were otherwise good machines. This could be because all the moving parts in 4 cylinder locomotives inherently are perfectly balanced. Therefore, the M Class would not have been rough-riding, unlike the XC. Many of them were painted in the green livery of the Bengal Nagpur Railway. Like all compounds, especially the 4 cylinder ones, their maintenance costs would have been much higher than those of regular Pacifics.
The de Glehn compounding system was the most advanced of all the steam locomotive compounding technologies. It enabled an engine to run on two cylinders while running light, or as a simple expansion engine while starting with a heavy load, or more efficiently as a 4 cylinder compound while cruising. Though it was widely used only in France, its originator, Thomas de Glehn was a Britisher of French and Prussian extraction.
In one of his books, E.S.Cox talks about his 'hair-raising' experience riding on the footplate at one of these engines. Because of some defect or faulty operation of the intercepting valve which directed the HP cylinder's exhaust into the chimney, a series of irregular muffled noises were produced which made the engine 'drave heavily'.
Hugh Hughes' scholarly work on Indian steam locos includes an impressive photo of an M Class Pacific.
DIMENSIONS
Cylinders: (2 HP) 16 1/2" Bore x 26" Stroke (2 LP) 25" Bore x 26" Stroke, Wheel Diameter: 6'-2", Tractive Effort: , Boiler Evaporating Surface: 2439 sq.ft, Superheating Surface: 637 sq.ft, Grate Area: 51 sq.ft, Boiler Pressure: 250 psig, Tractive Effort: 24,700 lb (as a compound), 37,000 lb (simple), Axle Load: 21.5 tonnes, Adhesive Weight: 65.4 tonnes, Total Engine Weight without tender: 105 tonnes.
