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It seems counter-intuitive, but one diesel engine in a locomotive makes it more reliable than two engines. Consider it mathematically: suppose a diesel engine fails unexpectedly once every 1000 hours of operation, and when it fails, it must immediately be pulled from service and go to the shop, where it requires 50 hours of repair, including transit time to and from the shop. Because the failures are unexpected, they are random. They will occur at exactly the same time only once in every 1000 failures (on a one-hour basis) -- or for all practical purposes, never at the same time. Thus we can reasonably expect the locomotive to be out of service 100 hours in every 1000. If the locomotive has one engine with the same failure rate, it will be out of service only 50 hours in every 1000. This is why the double-diesel was never a favorable solution and only existed when there was no practical way to get the desired horsepower in a single unit with one engine. This is also why the HEP set in a passenger locomotive is preferred over individual HEP sets on each car, and why a gearbox-driven HEP set is preferred over a separate diesel-generator set. The fewer engines you have, the fewer things there are to break, the fewer filters you have to replace, etc., etc. Your locomotive and cars will be available to do work more often. What the double-diesel DOES give you is some ability to limp in when one of its engines fails. But that is not the same thing as reliability. There is a point of diminishing returns: if the failure rate is high enough, you want to have two- or three- engine trains so you don't tie up the main line with dead trains. But today, locomotives have become so much more reliable than 30 years ago that one-locomotive trains are commonplace, even on busy single-track main lines. As Peter would tell you, steam-turbine driven warships often have two or three boilers (a high maintenance item) under the expectation that one of them will always be down for repairs or in standby mode for when the other fails. One boiler will be sufficient for cruising speed, and in fact, both boilers on line for full speed will deplete the fuel at four times the cruising speed speed rate, or worse. In a war emergency, you hope to have both available and on-line for high maneuvering speed. But even in World War II, the total number of battle hours in which a destroyer or cruiser needed all boilers on line was maybe four or five hours a month. In other words, the ship is designed for single-screw, one boiler, one turbine operation as "normal," and two-screw, two-boiler, two-turbine operation as "war emergency." In simple terms.
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