One note, your second claim is not entirely true - while land vehicles are a ways from being feasible to power with photovoltaics (although I would have to do the math for trains, which may be close), we already have primarily [1] and exclusively [2] solar-powered aircraft.
Those aircraft limp along at a tenth the speed of a typical commercial flight. That's the main reason I said "effectively". I mean, there are useless pure-solar cars too.
Trains might be closer but when single locomotives are measured in megawatts I'm not very confident.
Going with a weight of 20 lbs/m^2, that's only 600 lbs - less than 1% of the fully-loaded weight.
It's fairly likely one could get well below that mass using some of the more experimental solar panel designs (such as those in use for solar aircraft), but it would probably not be cost-efficient .
