XB 4-6-2

Click here for photo of XB.

Click here for photo of XB.  Bought from the Corbis Collection.

I once saw an XB accelerating out of Madras while hauling the West Coast Express in the early 1970s.  On another occasion, it was performing shunting duties at the Central Station.  The last time I saw an XB was at Tirupathi in 1983 when I was in my teens.  To my eyes, the XB was one of the most beautiful steam locomotives ever to run in India. 

Yet, it was also one of the most troublesome, as the Pacific Locomotive Committee Report describes in great detail.  True, its problems were, for the most part, ultimately resolved and it gave good service in its later years.  And it has the unique distinction of being the unwitting cause of reverse technology transfer between India and England.  For once, a solution discovered in India also proved useful to railways in Great Britain.

In the India of the early-1920s, steam locomotive technology was outdated.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 XB 4-6-2. 

TECHNICAL NOTES

When it was introduced in 1926, the XB Class was as up-to-date as any locomotive in England, having been designed by Messrs.Rendel Palmer and Tritton, a consulting engineering firm of London.  It had long travel Walschaerts valve gear and plate frame.  It had American-style 3-point suspension with compensating levers that was suitable for the indifferent tracks sometimes found in India.

Unfortunately, the XB was prone to frame fractures.  One specimen had 9 frame fractures in 9 years.  The firebox tubeplates had to be frequently changed due to cracking in the radius of the top flanges.  The XB's boiler was a poor steamer because the cross-sectional area of the boiler tubes was too small for the flue gas volume the firebox generated.

However, the worst fault of the XB was its tendency to hunt or oscillate laterally, i.e., at right angles to the track causing the track to distort!  Hence, the XB had a tendency to derail.  A terrible derailment occured in Bihta on the Eastern Bengal Railway in 1937 which took 100 lives.  XB Number 1916 which had been put on the head of the train in place of the usual 4-6-0 was travelling at 60 mph when it jumped the tracks.  The tracks themselves got twisted like noodles.

An angry public's demand for a judicial enquiry resulted in the formation of the Pacific Locomotive Committee which included 2 Indian engineers.  What followed was a classic engineering whodunit involving several colourful engineers which is well-described in E.S.Cox's book Locomotive Panorama and in the Pacific Locomotive Committee Report.  These engineers rode thousands of miles on several XBs to experience its sporadic rough ride.  It was a Frenchman Robert Leguille who came up with the solutions that eliminated the XB's derailing propensities.  He and Cox examined the flange forces measurements made by two British engineers and decided that the leading and trailing bogies needed much stiffer side control springs with better damping.  Experiments proved them right.  The XBs belonging to the BB&CIR and the M&SM railway companies were modified according to their recommendations and no more derailments happened.  The XBs in other railways were relegated to slow trains.  Many of them survived into the 1980s proving that they had become useful locomotives.

A postscript to this story is that the same recommendations proved useful in improving the locomotives of the LMS Railway in the U.K, which had exhibited similar problems though on a smaller scale.  For once, good engineering practice had gone from India to England and not the other way around.

DIMENSIONS

Boiler Pressure: 180 psig, Grate Area: 45 sq.ft, Cylinders: (2), 21 1/2" Bore x 28" Stroke, Wheel Diameter: 6'-2", Tractive Effort: 26,760 lb,  Nominal Axle Load: 17 tonnes, Engine Weight without tender: 90.2 tonnes.

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