Is there any technical reason they can't plate all sleepers with solar panels (or paint on a suitable substance etc.)?  If they also placed solar panels to either side of railway tracks, would the two (panels on sleepers plus panels either side of the track) generate significant power to the network?

The starry-eyed part is that I wish to see a non-polluting high-tech train network--at a reasonable cost--spreading across Europe and beyond.  And asap.

The boyish obsession part is that I see lots of "mostly not being used" track sitting there and think...what I wrote above.

Could they also (or instead of solar panels) place windmills either side of the tracks?

Don't fight forces, use them R. Buckminster Fuller.