This suggests to me that locomotives (particularly 20th century branch line variety) might be more adaptable than modern passenger stock. 1 in 25 grades and the length of the 5 chain curves might come into it too.
There are 5 chain curves, and steep grades on:
* Batlow lines.
* IIRC at least one 4.5 chain curve.
I seem to remember that QR had some 3 chain curves on one of the inland mainlands, from perhaps Rockhampton or Townsville.
The Oberon, Dorrigo, and Batlow were limited to 19 class locos in steam days, perhaps because of axleload considerations as well as wheelbase issues.
The Canberra line has one set of reverse curves with no intervening straight, and it was forbidden to couple two of the big (Speedo?) oil tankers together. Short reverse curves are also a problem with IIRC 1:8.25 turnouts on 3.66m centred double track. The newish crossover a P9 and P10 at Redfern are about 1:12 turnouts, which create a longer effective straight in the middle of the reverse curve.
Some of the earlier lines were built without transition curves between the straight and the pure curve. Transitions are difficult to add later, particularly if there are tunnels and bridges in the vicinity.
These days, the general minimum radius curve would be 10 chains (200m) on such things as triangles and balloon loops. Think of North Strathfield Triangle, or Islington Triangle(s). The Canberra line now has many 20m transition curves. but these days TC are say 60m, and even longer if line speeds are higher. Superelevation (cant) also has to be transitioned (ramped).
The original Blue Mountains line was apparently a series of 8 chain curves, most eliminated during duplication, except for the un-easable one at Zig Zag.
The Curve and Gradient books are somewhat incomplete:
* Separate curves on either side of island platforms are not always shown (say Belmore or Turramurra).
* Multiple tracks, such as the Relief lines through Broadmeadow are not always shown.
* Speed boards, depending on radius, transition curves or ramps, are not always shown.
* Only a perway surveyor has the detailed information to calculate what the line speed should be. (Ask perwaynut.