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The Quadruplex Locomotive was a planned train concept that wasn't built at all, though there was a quadruplex built in Belgium. In June 1914, George.R Henderson was granted US Patent 1,100,563 for a Quadruplex 2-8-8-8-8-2 locomotive, which was assigned to the Baldwin Locomotive Company. Baldwin submitted the design to the Atchison, Topeka, and Santa Fe Railway, which in the 1910s was a strong proponent of compound locomotives. This would have been, in 1913, by far the largest steam locomotive ever proposed. In Quadruplex form, it would have been 129 feet 10+1⁄2 inches (39.586 m) in overall length, a total weight of about 885,000 pounds (401 t), with a tractive effort of 200,000 pounds-force (890 kN).

The Quadruplex Locomotive was a 2-8-8-8-8-2, it was designed to have two boilers on it as going off of a single boiler was absolutely absolutely not going to be enough; according to the patent this was a compound locomotive, engine cylinders 7 & 9 would receive high pressure steam to drive the 1st & 3rd sets of driving wheels and the exhaust as lower pressure steam from cylinders 8 & 10 would power the 2nd & 4th sets of wheels; as well as the idea that the engineer's/driver's cab was at the front of the locomotive as it would be way too long to put it at the back, the fireman's cab was behind the firebox, so the crew were separated similar to the Camelback locomotive, he had proposed a communication system between the cabs that would use cables or rod operating devices (much like the engine ordered telegraph that was used on steam ships, or even a voice-pipe), The boiler would be jointed and have a flexible coupling so it can have it's own articulation on tight curves, the two boilers would be served by only 1 firebox, Henderson also included a turbine driven extractor fan that was within the smokebox that would be intended to maintain a constant draft of the flues in both boilers, that's because Henderson calculated that a conventional blast pipe utilizing steam from the low pressure cylinders wouldn't have been enough to provide a sufficient draft through the boiler while the locomotive was in motion, he clearly figured this motion out. But sadly, this Quadruplex locomotive was never built at all.

Belgium Franco-Crosti Boiler SystemBig text

The first Franco-Crosti boilered locomotive was built for the Belgian State Railways as No. 2096 in 1932, it was also known for being the world's only Quadruplex Steam Locomotive ever builtItalic text as No. 2096 in 1932. This locomotive had a massive wheel arrangement of 0–6–2+2–4–2–4–2+2-6-0, it weighed at 248 tons and had developed a power output of around 3,000 horsepower.

It was essentially two Franco-Crosti boilered locomotives joined back-to-back and was one of the most powerful articulated locomotives ever built. The locomotive was 31 metres long and developed about 3000 horsepower.

There was a water wall between the two fireboxes. The coal was stored in vertical bunkers, the bottom of which reached under the boiler to the fireman's position.

The boilers were slewed sideways so the fireboxes overlapped, presumably to meet some limit on overall length despite the articulation. Two firemen were needed; in fact, their use seems inescapable if only because of the firebox layout. This extra labour at a time when manpower costs were increasing must have cancelled some or all of the economies stemming from the Franco-Crosti system.

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