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Fuel always a concern for the railways..

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Overmod - I really don't know how removing sulfur affects lubrosity (?) (lubricosity?). I just remember it was a concern in the early nineties when low sulfur fuel first appeared. I don't remember any concern about rubber seals, and imagine any problem with sealing materials would more likely be caused by volatile components. At any rate, a high pressure system with metering and delivery done in the pump and injection done at a seperate injector like a Bosch or Stanadyne system has all metal to metal sealing. Some unit injector systems use O-rings to separate delivery and return sides like Cummin's PT injection, or Detroit's Series 50 & 60 electronic unit injectors, but this is for low pressure delivery in the cylinder head cavity and also a low difference in pressure from one side to the other of an injector. Whether metering and delivery is done in an injection pump or in a unit injector, the mechanism is the same - a barrel with a reciprocating plunger inside. The plunger has a short groove that connects the swept volume to a helical groove whose shape determines filling time as well as start and end of injection times. As long as the helical groove does not encounter the spill port, hydraulic lock pressurizes the fuel past the delivery check valve. Rotating the the plunger axially as it moves up and down changes the timing of its coincidence with the the spill port. This is done with a gear quadrant and a rack - fuel rack, controlled by the governor. These two pieces are not the only pieces with very close tolerances but are ones with the most movement. They must have a very close tolerance fit because of the pressures required for diesel fuel injection, and this is where the concerns for lubrosity arose that the service life of injection components would be affected. You could very well be right that any variation of lubricosity could be corrected with additives. Oh, the one system that is unique from the above description is the Cummin's PT injection system which, among other traits, is why I compare Cummins to Trolobytes (unchanged for millions of years), because the PT system has features that go all the way back to the earliest "solid" fuel injection systems of Vickers in the 1920's and even from earlier blast air injection systems when manufacturing tolerances did not make "solid" fuel injection possible !
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