And fter all of that, the whole thing is still highly illogical in the prototype world.
Allegedly, lesser used lines warrant less expense, but since the same weight of locos and rolling stock will be running over those lighter rails...hmm.
In the narrow gauge world of the late 19th century, it made absolutely no sense whatsoever, as the sidings frequently led to mines from which the return trains, full ore cars at that, weighed every bit as much as any train running mainline. In the old photos of the time, you can see the effects on the lighter sidings and branch lines as they took quite a beating, especially as they were often ballasted with local dirt to save time and money.
As for turnouts, it's a problem, because for mountain narrow gauge a #4 is protypical for sidings and branch lines, where space was often at a premium and didn't allow for longer easements or transitions. Again, you can easily see the abruptness of the turnouts in old photos. A #5 would be encountered only at a mainline junction, and none of the narrow gaugers ever went fast enough to need a #6 or higher. Average speed was often 20mph or less.
An even bigger problem is just finding wyes in whatever code you are looking for in narrow gauge, as the ever-popular turntable largely did not exist at all in mountain narrow gauge railroading. By far the most likely turnaround was the common wye, present virtually everywhjere except in locations where large engine houses were present. On the 40 mile stretch of the Florence & Cripple Creek RR, there was one at Florence, one midway around Adelaide to allow turning helper engines, and up to seven or eight in Cripple Creek and surrounding environs, served at its peak by no less than six railroads - four narrow gauge and two standard gauge - and one trolley car line. Not a roundhouse or a turntable in the whole bunch, with the exception of the extraordinary trackage at Corkscrew Gulch. In fact. the wye at Anaconda, midway between Victor and Cripple Creek, is still in use by the 2 foot gauge tourist train, the only working two footer anywhere around.