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The reference for locomotives and railcars

Bengal Nagpur classes N and NM

India 1929 26 produced
No. 814 on a works photo flickr/Historical Railway Images

After the Bengal-Nagpur had tested two class HSG 2-8-0+0-8-2 Garratts for slow and heavy coal trains on inclines, they ordered the 4-8-0+0-8-4 class N in 1929. With a service weight of 234 tons, they are the heaviest locomotives ever to run in India and were perfectly suited to well-laid and relatively straight tracks found on the BNR network. They could haul up to 3,000 tons on the level and even managed 2,400 tons on inclines of one percent.

To enable the operation on a wider selection of lines, the 16 class N locomotives were joined in 1931 by ten NM class locomotives which were similar, but lighter. To date, these 26 locomotives are the only Garratts ever built with a 4-8-0+0-8-4 wheel arrangement. Two locomotives of class N are still existing. While No. 811 has run a few times in 2006 and 2019 but has an unclear future, No. 815 is on display at Delhi Railway Museum.

No. 815 at the Delhi Railway Museum flickr/TrainsandTravel

Technical data

General

Property N NM
Built 1929 1931
Manufacturers Beyer, Peacock & Co.
Wheel arrangement 4-8-0+0-8-4T (Double Mastodon (Garratt))
Gauge 1,676 mm (Indian broad gauge)

Dimensions and weights

Property N NM
Wheelbase 8.38 m
Total wheelbase 25.63 m 24.82 m
Service mass 237.8 t 207.4 t
Adhesion mass 162 t 140.8 t
Rigid wheelbase 4.75 m
Axle load 20.6 t 17.9 t
Water capacity 45,459 l 22,694 l
Fuel capacity 14.2 t (coal) 8.1 t (coal)

Boiler

Property N NM
Grate area 6.48 m² 6.28 m²
Firebox surface 26.57 m² 30.658 m²
Tube heating surface 289.672 m² 244.428 m²
Evaporative heating surface 316.242 m² 275.086 m²
Superheater surface 59.644 m²
Total heating surface 375.9 m² 334.7 m²

Power plant

Property N NM
Driver diameter 1,422 mm
Boiler pressure 14.5 bar
Expansion type simple expansion
Cylinders four, 521 x 660 mm four, 508 x 660 mm

Power

Property N NM
Power source steam
Estimated power 1,715 kW (2,332 PS) 1,659 kW (2,256 PS)
Top speed 72 km/h
Starting effort 309.8 kN 294.9 kN

Calculated values

Property N NM
Factor of adhesion 1 : 5.13 1 : 4.68
Optimal speed 34 km/h
Power-to-weight ratio 7.2 kW/t 8.0 kW/t
Heating Surface Equivalent 568.4 m² 547.7 m²
Cylinder volume/HSE 1 : 989.68 1 : 977.62
Adhesive weight/Total 1 : 1.47
Tractive force/Heating surface 1 : 17.22 1 : 18.40
Grate area/Heating surface 1 : 58.01 1 : 53.30
Firebox/Tube surface 1 : 10.90 1 : 7.97
Total/Superheater 1 : 5.30 1 : 4.61
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