Duplex locomotive
The main disadvantage of the two-cylinder locomotives is the heavy hammer blow on the rails caused by the attempt to balance the reciprocating parts with additional masses mounted in the wheels.
[1] In Europe this problem was often overcome by dividing the drive between inside and outside cylinders, or else by using articulated locomotives, although at the time it was not believed possible to run one stably at greater than 50 mph (80 km/h).
As locomotives got larger and more powerful, their reciprocating machinery had to get stronger and thus heavier, and thus the problems posed by imbalance and hammer blow became more severe.
Ralph P. Johnson thought that the growing size and piston thrusts of existing express passenger locomotives could not be sustained with the by-then conventional 4-8-4 two-cylinder layout.
This had its own complications, and the challenges were sufficiently concerning for many roads, for whom current locomotives were taxing enough, to reject the duplex idea.
However, a group of enthusiasts is in the process of building a new Pennsylvania Railroad class T1 duplex locomotive, using design improvements from the post-war steam era in the hope of achieving better performance characteristics.
It was, in fact, too large to work over the majority of the PRR's system and was placed into service only between Chicago, Illinois and Crestline, Ohio (283 miles [455 km]).
In service after December 1940, it proved powerful and capable but prone to wheelslip and surging, presaging the problems with later duplex designs.
As soon as wartime restrictions on producing passenger locomotives were eased in February, 1945, the PRR placed an order for 50 production examples.
This was a fateful step, since the problems encountered with the prototypes had not been ironed-out nor had they been tested with the intensity required to be sure of production reliability.
A nonprofit group known as the T1 Trust is in the process of constructing a new duplex locomotive, a T1-class engine known as Pennsylvania Railroad 5550, intending to utilize design improvements from the postwar steam era not used or seldom tested on pre-existing T1s in the hope of creating better performance characteristics.
Like the B&O's George H. Emerson it had the second pair of cylinders facing backwards, and all were fitted with standard Walschaerts valve gear.
Production locomotives followed from the end of 1944, but these were rather different, the lesson that backward-facing cylinders next to the firebox were a poor design choice made clear.
With dieselization, they were the obvious first targets to be withdrawn since they were only a little more capable than the conventional J1 class 2-10-4s, but with far higher running costs and maintenance loads.
In France, the duplex type was made famous by the ten 2-4-6-2 compound locomotives built in 1932 for the Chemins de fer de Paris à Lyon et à la Méditerranée (PLM) to haul heavy freight trains on the 0.8% grade of the Paris–Marseille railway between Les Laumes and Dijon.
In a test on December 19, 1933, the engine developed slightly more than 3,000 hp (2,200 kW) at the drawbar over a distance of 37 miles (60 km) and a speed of at least 46 mph (74 km/h), without being overworked.
