When observing the merry go round trains delivering to our local power stations in the lower Trent Valley back in the 1970s, I came to the realisation that which of the three organisations involved would make a loss was actually completely academic, since the producer (NCB),the consumer (CEGB) and the carrier (BR) were all nationalised and so the Government of the day could arbitrarily make any one of them profitable simply by adjusting the price paid by the CEGB to the NCB for digging out the coal and to BR for moving it
I'm not sure actually whether the CEGB or the NCB paid for carriage - it would depend on when the coal became CEGB property - although I do know that the wagons were weighed at both ends of the journey.
During a summer job at one of the power stations when I was a student, I was able to solve a puzzle that had baffled the regular staff as to why one of its weighbridges was intermittently weighing a phantom wagon. (There were two weighbridges, one before and one after the unloading bay, the difference in weight they measured being the amount of coal delivered). Each weighbridge took a measurement when a pair of sensors, spaced at the wagons' wheelbase, both detected an axle. Asked to monitor what was happening, I was able to confirm the manager's suspicion that it was also sometimes taking a reading when the rear axle of the loco and the front axle of the first wagon were over the sensors. But because I had been noting the loco numbers for my own interest, I realised that it was not random - it always happened when the train was hauled by a Class 56 - and never with a Class 47. It so happened that the position of the rear axle of a 56 relative to the lead axle of the first wagon was close enough to the wagons' wheelbase to trigger the sensors, thereby weighing approximately one sixth of a locomotive and half a wagon. The other weighbridge had the sensors marginally further apart, so the problem didn't occur with that one. The solution was to tighten up the tolerances on the positions of the sensors.