Well... just for the heck of it, let's take a 100 km high-speed railway with two tracks, no tunnels or bridges or walls to the South, with two trains per hour per direction running with an average power of 5000 kW and no significant regenerative braking, each train taking 30 minutes. That makes an average power need of 10 MW. Taking a central European yearly average capacity factor of 11%, a further reduction by a factor of 0.7 due to laying the solar panels flat rather than inclined, we'd need a generating capacity [e.g. nominal capacity] of around 130 MW. You get solar panels with 80-140 W/m² nominal power, the larger the higher, due to the small width of sleepers let's go with 100 W/m² -- hence you need to cover 1,300,000 m². If I'm generous, you can consider fixed track instread of track on sleepers, and you have a suitable area of 1.3 m² per metre track between the rails, that is 130,000 m²: only a tenth of what is needed.

The above was all just a theoretical exercise. In practise, due to the vibrations, whipped-up stones, and dirt falling off trains (if you can keep oil in check, there is still snow and powder from brake pads), solar cells have no chance of surviving in tracks, or over a bit longer time even on the ground near tracks.

Wind power would be much more likely to have the potential: you'd only need 16 turbines of 2.5 MW each to give the needed average power, and with four times as many, you could cover the power need (with some to spare) most of the time.

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