Switzerland is, bluntly, railfan heaven. Nowhere else on earth has such a high concentration of railway systems, nowhere else relies on railways so much as a crucial part of their nation's transportation infrastructure, and certainly nowhere else in the world has so many narrow gauge railway systems. Not just for tourist service, either, like those of Wales or Colorado, but active railway systems used for freight as well as passenger service.
They range from tiny lines that just run from one small village to another small village, to larger systems that provide the main transport system for an entire Swiss Canton, such as the RhB.
Most are built to metre gauge, in other words with the distance between the rails being 1000mm, or approximately 3'3". This enables a reasonably large loading gauge without being too unstable. It's a good compromise; the tighter minimum radius allowed by the narrow gauge allows the lines to hug the mountainous Swiss terrain rather than requiring heavy civil engineering to alter the terrain, as a standard gauge line would require. This all makes for cheaper construction than standard gauge, without being too small for serious purposes.
These days, pretty much every line is electrified, Switzerland being ideally suited to hydro-electric power generation. Swiss railways in general went straight from steam locomotives to electric locomotives; very few diesel locomotives were ever acquired by Swiss railways.
This is a (possibly incomplete) list of Swiss narrow gauge systems, and their abbreviations: