Even other countries don't prefer Nuclear energy for small scale applications. The first idea of a nuclear powered train was mooted sometime in the 1950's by Russia. At that time there wasn't much awareness about the danger posed by nuclear reactors ( disasters like the Chernobyl and Fukushima occured much later).
Portable reactors were developed for research stations in Antarctica, but while these reactors were small ( hardly 1 metre x 1 metre in area), but they were not designed to be put on moving machines. More than reactor size, the problem is the shielding from the generated radiation ( which has a really high penetration power). And when people are walking beside the loco in...
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the stations and in the maintenance yards, they will be basically risking their lives. And a little mistake in handling the Uranium within the loco, will result in a disaster. In other words, nuclear powered trains will be like moving bombs!

As far as I could search, I've found no mention of Solar -powered trains other than IR's trials with Solar powered coaches. The only thing I could find was a discussion on using Solar power to super heat water in fireless steam locomotives.
Usually our locomotives consume power on the order of Mega watts. I doubt if consistently meeting such power levels is possible with just the area available on the shell of a locomotive. And since, the output of the panels is affected by the surroundings and weather also, consistently meeting the power requirement of the locomotive will be very difficult, even more so on gradients etc. Even for using simultaneously with the OHE power,...
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the design will become extremely complicated as the OHE will be on AC power and Solar Panels always generate DC power. The additional equipment required for regulating the supply form the Solar panel will eat up a lot of space on the locomotive. And the most frustrating problem will be using the Solar power and the OHE power together as a lot of care has to be taken before connecting any power supply in parallel to a power grid.
However, on-board solar power might be a good option with battery operated shunting locomotives, which can haul a battery car fitted with solar panels to recharge the battery.
A more easy alternative is a fuel cell locomotive using hydrogen fuel cells to generate energy. These will generate very tiny amounts of Carbon dioxide as compared to diesels and the best part is that all the existing diesel locomotives can be retrofitted with fuel cells after removing the diesel engines, saving on the development cost and time. Some research has been conducted on this in IR ( No functional locos though). You can see a mention of it in this document : click here