ROI would take a lonnnnng time, in my view. It's the same idea as installing modules under asphalt roads or even above them. It's not really worth the O&M hassle whatsoever.

The same idea goes for installations like canals. They're linearly too so not the wisest use of modules. But the benefit of modules over water like that is lower rates of evaporation as the panels block the sun. Same is true for water department reservoirs that see lots of algae. The panels block the sun and choke out the algae.

But you're right about the shading. If these panels are installed in higher latitudes, then odds are they might never see the sun directly at certain times of the year. Usually solar designers only recommend flat horizontal mounting for modules in hurricane- or tropical storm-prone regions like near the tropics, or close to the Equator where the Sun shines directly overhead most of the year.

What I COULD see happening is if governments around the world start transitioning railroads to have H-frame structures that suspend feeder lines like what's used in electric trains with pantographs. If you set up those H-frames frequently enough, you have the underlying structure similar to carports and can install modules 4-10 modules wide. THEN you can utilize string inverters every 7-8 H-frames or so, converting the solar DC power into AC which can help feed the train loads as they pass or feed the grid.

Bonus of the above system is that over time, all trains including rail freighters become electric or at least hybrid to make use of the free power generated above them throughout the railway.

Lots of ideas!