Solarpunk technology

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.