The problem for narrow gauge railways I know of was not motive power: due to smaller amounts to transport, that was not a big issue. The problem is the need to reload: if you have to reload into a standard-gauge wagon after just 20 km, why not transport it on trucks all the way? So surviving narrow gauge with freight transport usually carries normal freight wagons piggyback (i.e., either on special flat cars that have normal gauge rails on them, or on special bogies that are pushed under each axle of the normal wagon).

On the other hand, some narrow gauge railways live on fine as regional transporters or tourists: in Switzerland, there are whole networks.

As for Japanese rocket trains: thanks, never heard of this! I only know that the absolute high-speed record on rails is held by rocket sledges at a US rocket test facility. In 2003, the record was increased, it now stands at Mach 8.5 / 6416 mph / 10325 km/h.

_{*Lunatic*, n.
One whose delusions are out of fashion.}