A very neat idea, quite innovative! My initial reaction would be that this would be a great way to recycle panels from larger, traditional projects that are getting upgraded anyway. My concern with new panels is that with moving trains above them, there's a strong chance for breaks in the glass depending on what's being transported, and a huge amount of loss due to dirt accumulating. Those conditions may result in harsh losses for new panels, which may be more beneficial in production elsewhere! Mid- to end-of-life panels might not be producing as much power anyway, so getting a second life in a situation like this one might squeeze a little more power out of them that you wouldn't get otherwise.

Of course, solar panels are very inexpensive nowadays, so financially it might not make that big a difference, but I think it's good to think about best use of the resources we have regardless!