For poor areas of the world where there is none or scarce transportation, how about physically smaller trains about 1/4 size like the RR in England. Main use would be for passengers. Lower costs, lower weights of trains would need cheaper bridges, narrower right of ways.
As the early narrow-gauge builders in the US and elsewhere discovered, while the cars might have 1/2 the capacity of their standard-gauge equivalents, their construction and operating costs weren’t 1/2 that of standard gauge. In fact, they were almost as much as if standard gauge had been used in the first place.
Japan had quite a few kilometers of 30 inch gauge, in the form of little country tramways, logging railroads, mineral carriers… Almost all have dried up and blown away. I rode some while they were still around - the fares were at least as high as those for comparable distance on the National Railways ordinary passenger trains. Even so, it was obvious that they weren’t making enough to keep up with expenses. Only one of significant length survives - to serve the logistic needs of a hydroelectric development in a gorge that has never seen a road. It carries tourists as a sideline.
I remember searching out the faded remnant of a once-extensive system. About once an hour, a four wheel ‘critter’ dragged a dilapidated passenger car from one end of the line to the other - a distance of about five kilometers. The company also ran all the busses and taxis in the area. Since the name on all those rubber-wheelers was that of the railway, the local government insisted that they had to have at least a little bit of railway in operation. (The busses were crowded. That sorry little coach wasn’t.)
Chuck
The world’s capitol for narrow gauge is probably Switzerland, with South Africa a runner up. The Swiss have a lot of meter-gauge very modern and well-used mileage, most of it in the Alps, but some more like classic interurbans than railroads (including street running with freight and mixed trains). There is quite a continuous network between Tirano, Italy, north to San Moritz and Chur, and then west to Zermatt, with branches, and connecting with the standard gauge network at several points, basically Chur. South Africa’s railroad system is quite extensive, most modern and electrified, and nearly all three-foot-six-inch gauge. The rolling stock is actually wider than British standard gauge equpment. They had some pretty impressive Garratt steam locomotives. And I got to run a 4-8-0 at the Capitol Park Engine Sheds in Praetoria! Just from a siding to the sand and coal docks.