Solar panels have always bothered me because they require such intense manufacturing and usage of scarce resources. TLDR of article: “Many people will argue that if low-tech solar panels are less efficient, we would need more solar panels to produce the same power output. Consequently, the resources saved by low-tech production methods would be compensated by the extra resources to build more solar panels. However, efficiency is only crucial when we take energy demand for granted, sacrificing some efficiency may gain us a lot in sustainability.”
Transmission does not have excessive losses. They’re typically well under 5%. Even in grids with lots of long distance transmission (NZ has lots of generation in the south island, demand in the north island), the last-50-km losses are higher than the long-distance losses.
Rail current draw tends to be very bursty including both consumption and regen, with capacity that is quite limited on longer lines. The amount of power that can be connected isn’t very large, and because it’s a very long, narrow corridor, the average distance from a panel to an inverter is going to be large, or the inverters are going to be small.
Railways also like to regularly shut off the power to make maintenance safer and faster, and just use other tracks or diesel trains. This would interrupt the generation.
Mechanical mounting and windage concerns could also be present; a structure overhead could affect the aerodynamics of particularly high-speed trains, and the roof could funnel crosswinds into the train rather than over the top.
I’m not sure it’s impossible, it’s just that there are still much better places to put panels.