Bengal Nagpur classes N and NM
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.
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 |