Wouldn't work even on a bright day. Just look at the power some fast
trains generate at the rail:
TGV Paris Sud-Est: 6450kW
TGV Atlantique, -R, Duplex, AVE and Thalys: 8800kW
TGV Eurostar: 12200kW
ICE/V (410/810): 5600kW cont, 8400kW max
ICE1 (401/801-804): 9600kW cont, 15200kW max
ICE2 (402/805-808): 5000kW cont
ICT (411/415): ~12000kW cont
ICE2/2 (403,404,406): 8000kW cont.
I don't have any figures for the various models of Shinkansen.
At best you'd have about 10x3 metres available on the roof of each car.
Assuming a 2+8 formation train, that means about 300 square metres
available for photovoltaic cells.
The University of New South Wales currently holds the world record for
independently verified silicon solar cell conversion efficiency of 24%, an
advantage in performance of nearly 8% over the nearest rival. Under the
global AM1.5 spectrum of 1000 watts per square metre, this means you'd be
able to generate about 72kW from the roof of a train. This is 0.81% of the
power required to drive a new generation TGV up to 300km/h, not counting
losses during the inversion to three phase AC.
Cheers
David