In the department of Cher Napoleon established a vast forest of Oak. The purpose was to have materiel ready to built ships for his navy. This was a strategic resource, the same way steel, tungsten, etc. are today.

With the changes in shipbuilding, at least for naval vessels, the need was diminished for timber masts and planking.

The city of Montluçon had been a producer of cast iron for a long time. Canals are an ideal way to transport that long distances.

The oak forests shifted to being a source of charcoal both for the charcoal itself and for the production of cast iron. The ‘pigs’, ingots of cast iron, could later be wrought or further refined into steel.

Pretty much the tl;dr is that demand dropped off, because Oak was no longer needed for war and higher quality iron could be sourced more cheaply from the north and east, with better connections. It was thought that upgrading the canal was not cost effective, so it slowly died.

The canal itself had issues, it often suffered from insufficient (water) influx, and leaks. It was not terribly well waterproofed as the area is low in clay soil. It’s quite lovely through many parts, though there are some dry areas. The Locks still exist, and many of the Lock Keepers houses are there. They were all built to the same plan from Oak and stone, with lovely Tomette (hexagonal terra cotta tile) floors in the kitchen.

There is of course more, where it links (it doesn’t exist in a vacuum), the general depopulation of what the French call the ‘Diagonal Void’, and probably other important things that i will remember next week and think ’ that’s important, why didn’t I write that down? Now they think I’m an idiot’.

Because narrow gauge is always the odd man out. If you used narrow gauge (and there are many, the term encompasses quite a few different sizes) there was a reason. Could be something like tight turn radii, or as simple as bigger costs more.