M I remember reading in a publication some years back (& think its one item that is a large missing bag of mine following a move, that lost a lot of my docuementations, photo's & records) that the initial order was for 100 boilers suitable as you say for both 35 & 36cl locomotives, as the 35cl boilers were approaching their use by date, that would make the two classes more akin to the same, especially as both worked the same load & conditions on the Northern line.For A6ET, there are no brand new, unused 36 Class boilers in existence anywhere that I know of. All of the spares at Thirlmere, Eveleigh, Cardiff, Junee, Canberra and Dorrigo saw active revenue NSWGR service in one or more sets of 36 Class frames before becoming stationary boilers. Every one of them has a boiler number confirming same and I am sure that a detailed look through the 36 Class history cards will confirm this.
As for other ex-traffic locomotives falling by the wayside, every one of those that no longer steam are still and quite because they need varying amounts of work done to them to return them to traffic condition. In the past (pre-Shirley era) this was disguised by locomotives being taken into Eveleigh for 'remedial work' to keep them going. The cost (financial and labour) is what has stopped that happening now. Similarly, today's changed railway actively works against anything not capable of running at 80 km/h.
Back to the locomotive in the title, it is good news that a way out of the previous morass has been found and a fully Australian Standards compliant 3801 will grace the rails again. 245 psi too, so it will bark quite crisply and loudly, exactly as intended when designed in 1938.
Regarding 36 Class boilers, the contracts list in Railway Transportation magazine describes these as "35/36 class boilers" and the number contracted for in two separate contracts exceeded the 73 36 class fitted with these boilers. Are you saying that only 73 boilers were delivered, or that more than 73 boilers were installed in 36 class locomotives through their life?
As to 3801's pevious boiler now to be repaired, if it can at reasonable cost be modified to allow operation at 245 lbf/sq.in. one wonders why this wasn't done some time in the last thirty years when it was running at 215 lbf/sq.in.?
Again, if the previous 3801 boiler can not only be repaired at reasonable cost but that this can be acheived in less time than modifying the new German boiler to meet requirements that should have been in the design contract, why wasn't this done at some time while that boiler was on its trip to Germany and back?
In Thompsons 36cl book, the boiler that was fitted to 3642 was numbered 3646b, so would that mean, the boilers numbered at least 121, not counting potentials for the 35cl?
Your questions re the repair now of 01s boiler is exactly what I asked as well, in regards to the German boiler, the question really is why was it not built &/or repaired correctly in the first place after being sent back?