As many locomotives have the fuel tank below the frame in between the truck assemblies when they were once filled at an engine terminal or fuel rack but was phased out over environmental regulation. Now the locomotives are filled by a fuel vendor as the fuel bill takes a chunk out of the railroad's operating expenses. Once in the tank as those would wonder how the fuel gets to the prime mover as most diesels have some form of lift pump to push the fuel through a filter assembly and or water separator before making its way to the injection pump where its pressurized to be transported to the injectors as the prime mover uses more fuel for combustion than what is required as it helps cool the injectors before returning to the tank through the return hoses.
Take your meds and everything will be fine.
With modern injection pressures around 30 k psi to open the injector itself the fuel in excess is also needed to help keep the injector clean. Plus the return fuel helps cool the injectors off and the heat carried off by them is useful in the winter to prevent fuel gelling a major issue with ulsd it gells at a higher temp than the older stuff.
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