"Articulated" mallet compound locomotive

The following is taken from The Locomotive Magazine Volume XIV, January-December, 1908.


This engine is especially designed for working heavy goods trains on the Hedjaz Ry., on which there are long gradients with curves of 4½ chains, radius.  The trains to be pulled have a weight of 250 tons.  The permanent way being laid with rails of small weight, and the gauge of the track being 3 ft. 6in. only, it was necessary to have the heavy boiler carried by six axles.  The wheels of these axles are arranged in two groups of three pairs each.  The wheels of the hind group are six-coupled and placed in the main frame, to which the boiler is rigidly connected; they are driven by the two high pressure cylinders.  The wheels of the front group are in a bogie frame and the second and third pair are also coupled and driven by the low pressure low-pressure cylinders, while the leading pair of wheels have further lateral play provided in addition to that permitted by the bogie.  The arrangement of the wheels allows the engine, though the total wheelbase amounts to 28 ft., to travel around curves of 4½ chains’ radius with perfect ease.  The slide valves are actuated by Heusinger’s gear by means of screw and wheel.

The coupling of the bogie with the main frame differs from that hitherto provided in Mallet engines, being affected by a double joint, which requires less lateral play of the truck.  This coupling is arranged in the same manner as that between engine and tender.  The oscillating movement of the front group of wheels is controlled by springs and buffers.  The engine is fitted with the automatic vacuum brakes to all coupled wheels and tender wheels, with Gresham and Craven’s steam sanding apparatus and with Coale pop-safety valves.

The engine was built by Messrs. Henscel & Sohn, of Cassel.

The following are the principal dimensions:-  Engine:  diameter of 1.p. cylinders 20 in.; piston stroke 22 in.. diameter of coupled wheels 42? in. diameter of truck wheels 28?. ; working pressure 170 lb. per sq. in., heating surface 1,780 sq. ft., grate area 27 sq. ft., wheelbase 28 ft., number of boiler tubes 200, weight in working order 51 tons 12 cwt., weight on drivers 45 tons. Tender:  tank capacity 4,000 gallons, bunker capacity 5 tons, weight in working order 37 tons 16 cwt.

Articulated mallet compound locomotive, Imperial Ottoman Hejaz Railway.

The Hedjaz Railway when completed will be a remarkable work, traversing the desert between Damascus and the Holy City of Medina, a distance of about 800 miles through a country of religious fanatics and with serious natural obstacles in addition.  The leading stepping places en route are Ma’an, which was reached in May, 1905 about 250 miles south of Damascus, Tebuk 145 miles further, EL’ Ala 185 miles beyond Tebuk, Medain Saleh 600 miles south of Damascus, and Medina itself, which it is hoped to reach by September 1st of this year.  Water is scarce throughout the route.  South of Ma’an the railway has to descend from a tableland about 3,700 ft. above sea level down a steep escarpment into a valley 500 ft. below, by means of a long looped curve with a minimum radius of 336 ft. and with gradients ranging from 1 in 55½ to 1 in 62½, followed by a series of less severe curves and gradients, and beyond Tebuk a similar rise has to be encountered.

There are engine sheds and repairing shops at Ma’an and Tebuk, which will no doubt be added to after the completion of the line, and a feature of the establishment of blockhouses for the defence of the railway officials from fanatic tribesmen.  At present, the Ottoman Government forbid any European except the railway officials from going south of Ma’an.  Moslem engine-drivers and mechanics are almost exclusively employed, mostly drawn from the Turkish Navy, and of indifferent acquirements.