WABCO's battery unit that was used in the recent "hybrid" consist tests had 44 megawatt-hours (mwh, energy storage capacity) of batteries, powering a standard 4400hp AC drive. One takeaway from the test is that they need 60 mwh to work around a few issues that developed. Anyway...

Covering the top of that locomotive with solar panels would produce 4-5 kilowatts (kw, power) based on the usable square footage (similar to that of a small house). 4400 hp (granted, not produced all the time) is about 3.3 mw or 3300 kw. Hence the monstrous battery to supply it.

If you just left the locomotive out in the sun without using it, with the solar on top, the panels would produce about 40-45 kwh/summer day. To make the math easy, let's assume 44 kwh/day. With a 44 mwh battery, it would take ... 11,000 days to charge the battery from zero to full. IOW 30 years. Obviously, if you plan on recharging that battery in hours or at most a few days, you need more power than covering the roof space of the locomotive can provide.

Direct solar-driven travel, without battery backup, can work in demonstrations with purpose built vehicles that no normal person would be able to use. IOW technology demos or toys. To do anything usable for real life requires much more "roof space." There's a reason why commercial solar generation on a megawatt (mw) scale covers tens or hundreds of acres.