It is fairly clear that the VR initially believed that three S class would be enough for both the Sydney Limited and the Albury express, with one on each train and one spare. It was clear that much more maintenance was needed than initially expected, so a fourth locomotive was required and ordered separately later. Early VR publicity clearly refers to only three S class as being needed for the North East.
Even this was not enough and H220 as the only other locomotive capable of working the train, was drafted in on occasions when only one of the four S class was serviceable because of scheduled maintenance coinciding. This didn't happen often, and the H lost time compared to the S...
I personally believe that the three cylinder locomotive with the Holcroft-Gresley valve gear was as much the result of misleading publicity and sales pressure as could ever be attributed to EMD, although most of this came from Alco who obtained the US rights to the design. Holcroft warned Gresley about the theoretical deficiencies of this version of his design at the meeting when he explained the design to Gresley in 1918 - basically that valve settings would be hard to set correctly because the valve stems on the outside gears expanded in the opposite direction to that on the centre valve. A result was that the centre cylinder provided more power than the outer cylinders and overloaded the centre big end. The 1930 trials of NSWGR 5711 appear to show this on the few tests that the centre indicator was working... The valve gear used on H220 actually existed before Gresley adopted the Holcroft design and was used on many more locomotives, even counting the American designs of the 1920s. The three cylinder design increased the cost, compared to the two cylinder 600 class of the SAR which were otherwise equivalent.
I believe the delay in building the B class was nothing to do with EMD but was due to the Federal Government withdrawing the allocation of US Dollars, preventing the purchase of the the engines, generators and control equipment that was to be imported. It was at this stage that the NSWGR withdrew and ordered locomotives from Canada (where US dollars were not required) and the SAR decided to build their own locomotives with English Electric equipment.
Certainly, EMD drawings exist of early versions of double ended locomotives dated as early as 1949, complete with metric dimensions to assist EMD's European partners who were eventually to build locomotives built to tighter clearances but to the same general design.
It is interesting that VR didn't provide two B class for the Spirit of Progress since two B class were regularly assigned to the Overland well before they were used regularly on the SoP. I've been told that if the train was not at maximum loading, the B class could maintain the SoP timing, and it was on days of heavy loading that the delays occurred. Clearly two B class could have kept time with the heaviest load, and there must have been a conscious decision by VR authorities to allow the delays rather than to tie up another B class.
While the B class could produce 1500 HP into the generator from a standing start, a steam locomotive like a S could produce 2000 HP or more but only near its maximum speed. So the B could accelerate faster and would climb grades faster, but would be unable to run as fast on level track. The S also had to haul its own tender, which full of water and coal would be heavier than an additional passenger car.
The additional power in the S class diesels allowed them to maintain the SoP schedules even though they couldn't deliver the same power at high speed as did the steam locomotives.
Thanks for that, that tidies my musings up a lot, although in my head I knew why they needed a fourth engine, I did not notice I forgot to mention it.
Don't forget though that they nobbled the B Class by preventing them from accelerating off as quickly, initially. I seem to remember that the new circuitry dropped out about 15 MPH, and then the B would haul away. That initial start off took minutes out of a fast pass time table.
I think B67 had the TZR's removed in the 70's after breakdown, anyway whichever B it was ,it was a pleasure to drive out of Albury on the morning pass. I think the circuits were all removed by West Coast and other private owners in later years