Iirc this was a huge deal in the UK. A lot of smaller rural communities, mining companies, etc, used a narrower gage, especially in mountain regions where the path had to be put around or through mountain cliffs. You could also transport smaller pieces of equipment directly on tracks. Smaller engines could be used by more people to transport smaller loads more quickly than by cart

Also, keep in mind that railway routes need to maintain as close to level as physically possible, so if existing terrain doesn’t let that happen, your only option is an expensive bridge or tunnel.

Meanwhile, the open countryside and cities are using wide gauges suited to larger loads and the more open environment.

All well and good, until the meet. Changing cargo over is a large expenditure in both efficiency and profits. And then people start wanting to run the big lines where the smaller lines are. And the smaller lines also want to utilize the existing wider gage lines. So a push to standardize came out. But when the implication of a switch is tearing down and rebuilding bridges, or widening tunnels where it’s expensive/risky to do so, the debate becomes pretty nasty. A lot of money was on the line for the losing side, so it became a territory war. So they got together and largely standardized new rails being built.

Edit: Also, a huge reason for this push was that tourism and “taking a day-trip to the countryside” was exploding in popularity. Big cargo cars got converted into passenger cars to accommodate the sudden surge in passenger vs cargo travel demand. Horse-hair seats were used early on when wooden benches were deemed too uncomfortable. They shed oil and water easily, so were easy to maintain. These seats were the precursors to our modern bus and metro seats.

I watched a few documentaries years ago, so please correct me if im wrong. I have no idea how I keep this shit in my head but can’t remember what my boss asked me to do three days ago. Fam do I have the AuDHD?