The usual motivation was to halve the stresses in the connecting rods that communicated the power of the cylinders to the driving wheels. In a conventional locomotive there is on each side one connecting rod driving one crank, and the other wheels are driven from that one by coupling rods. Clearly there is something of a bottleneck as on each side all the power of the engine has to go through one connecting rod and one crank-pin, and as US locomotives grew in size and power this was becoming a problem. Another issue is that the connecting rod has to be of massive construction and so is difficult to balance.
There is however, a rather obvious snag to this concept. Wheelslip can now happen in two ways rather than one, and this was a serious problem with most if not all of the duplex designs. To see how destructive wheelslip can be, consider the Blue Peter accident in 1994. There is a rather grim video of the accident on Youtube.
THE PETIET LOCOMOTIVES: 1863
Jules Petiet of the Nord railway in France produced two duplex-drive designs in the 1860's.
The first was a tank engine in 1862 with independent driving axles at each end but three supporting axles in the middle; I suppose it would be called a 0-2-6-2-0 configuration.
A later design of 1863 had two sets of independently driven axles making it an 0-6-6-0.
These locomotives are fully described on the page of Petiet's French experiments.
THE LNWR TEUTONIC LOCOMOTIVES: 1882
Left: The Webb Teutonic class: 1882
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THE JAMES TOLEMAN LOCOMOTIVE: 1892
Left: The James Toleman locomotive: 1892
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Left: The James Toleman locomotive: 1893
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THE SAXON RAILWAY XV HTV LOCOMOTIVE: 1916
Left: Model of the Saxon Series XV HTV locomotive: 1916
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It is described in detail on the Klien-Lindner page, which also has the only known photograph, regrettably of poor quality. The picture here is of a model, builder unknown, which gives a better view of the construction.
The design has a Wikipedia page, which gives full technical specs.
THE PLM RAILWAY 151A LOCOMOTIVE: 1932
Above: The PLM 151A locomotive: 1932
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The low-pressure cylinders drove the first set of coupled axles, and the high pressure cylinders the second set of coupled axles. The front and back crankpins were set at 180 degrees so the back and forth forces from the movement of the pistons was balanced out. The 151A was therefore a balanced locomotive.
Most importantly, the two sets of drivers were linked with inside connecting rods and inside cranks on the 2nd and 3rd driving axles, so despite having separate groups of cylinders these locomotives were not true duplex, but actually 2-10-2, which probably explains their success. This arrangement is apparently sometimes called 'conjugated duplex'. Certainly the great Andre Chapelon considered them to be of the 2-10-2 configuration, and I don't plan to argue with him. The driving wheels had a diameter of 1.50 m (4' 11"). The rear cylinders were angled upward to reduce the distance required between the second and third driving axles. The cutoffs for the LP and HP cylinders were linked so the work done was in the ratio 2:3, since the LP cylinders drove two axles directly but the HP cylinders three. This minimised the stresses on the internal connecting rods.
Above: A PLM 151A locomotive: 1932
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Above: Section of the PLM 151A locomotive: 1932. Dimensions are in millimetres
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The locomotive can be found on the Wikipedia page for Duplex drive.
The 151A has its own Wikipedia page but it is in French.
THE BALTIMORE & OHIO Class-N1 4-4-4-4 LOCOMOTIVE: 1937
Left: The George H Emerson locomotive No 5600: 1937
Power output: 3936 hp Weight of loco: 175 tonnes |
Left: The George H Emerson locomotive No 5600: 1937
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Left: The George H Emerson locomotive No 5600: 1937
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The design was not very successful. Putting the rear pair of pistons beside the firebox meant the locomotive was kept to the same coupled wheelbase as the B&O's current 4-8-2 engines, but space there was very limited so there were compromises in firebox and cylinder size. The piston and valve rods also suffered from grit from the firebox. It was tested until 1943 and then withdrawn. It was not repeated.
The PRR N1 locomotive has a short Wikipedia page that gives the full specs. The locomotive can also be found on the Wikipedia page for Duplex drive.
THE PENNSYLVANIA RAILROAD Class S1 6-4-4-6 LOCOMOTIVE: 1940
Left: The Pennsylvania Railroad S1 locomotive: 1940
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Left: The Pennsylvania Railroad S1 locomotive: 1940
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Boiler pressure: 300 psi
Power output: not available
Weight of loco: 276 tonnes
Left: The Pennsylvania Railroad S1 locomotive: 1940
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The S1 was taken out of use in 1945 and scrapped in 1949.
The PRR S1 locomotive has its own Wikipedia page.
THE PENNSYLVANIA RAILROAD Class T1 4-4-4-4 LOCOMOTIVE: 1942
Left: The Pennsylvania Railroad T1 locomotive: 1942
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Boiler pressure: 300 psi
Power output: 6550 hp (Test of No 6110 in 1944)
Weight of loco: 224 tonnes
All T1's were out of use by 1952, and were scrapped between 1951 and 1956, but the T1 Locomotive Trust is planning to build a new one, with the goals of running a mainline steam excursion service, and- wait for it- to set a new World Speed Record for a steam locomotive.
Left: The Pennsylvania Railroad T1 locomotive: 1942
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The T1 has a Wikipedia page, which gives full specs.
In this Youtube video there is a frightening example of T1 wheelslip at 1min 22 seconds.
THE PENNSYLVANIA RAILROAD Class Q1 4-6-4-4 LOCOMOTIVE: 1942
Above: The Pennsylvania Railroad Q1 locomotive: 1942
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Also unusual for a goods locomotive was the partial streamlining, and the bullet nose rather than a conventional smokebox door.
The Q1 was a one-off experiment and It was described as "having all the drawbacks of the B&O loco". These included the location of the rear cylinders; they compromised firebox size and were subjected to grit and dust. Furthermore the long pipework caused thermal losses. It remained in intermittent service (it required a good deal of maintenance) until late 1946, when it was withdrawn and put into storage, and dismantled around 1949. It was the basis of the PRR Q2 series.
Boiler pressure: 300 psi
Power output: not available
Weight of loco:
The Q1 has a Wikipedia page, which gives full specs. (except the power output)
There appear to be no Youtube videos of the real Q1 in action, though there are a few of model Q1's. Here is a fine example.
THE PENNSYLVANIA RAILROAD Class Q2 4-4-6-4 LOCOMOTIVE: 1944
Above: The Pennsylvania Railroad Q2 locomotive: 1944
Power output: 7987 hp Weight of loco: 276.4 tonnes |
The Q2's were considered a successful design, but did not last long in service. They were not much more capable than the conventional 2-10-4 PRR J1's, but had much higher operating costs and greater maintenance requirements. As a result they had all been withdrawn by 1951, though the J1's carried on until 1958-59.
The Q2 has a Wikipedia page, which gives full specs.