energy density is too low for solar.
The RMPU for AC itself require 25kW, and there are two of them, one on each end of the coach.
add into it another 10 kW of lighting and small power load and your power requirement becomes 60 kW. for those 60 kW, using monocrystalline type panels (which give highest efficiency, but are very costly and have to be imported) you need an area of almost 5500 Sq. Ft. to realize how much area that is, consider this: total surface area of AC Coach is around...
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800 Sq Ft.
Unless some major innovation doesn't happen in solar PV industry, this will never come to pass.

1) But right now without Deisel Gensets we don't have any efficient way for feeding power to LHB coaches na..?
-- We do, we can do it easily by using HOG Loco and rake modified for HOG operation.
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2) a small boy thought of using wind fan in travelling train to produce Electricity..
--That...
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idea is quite childish, as the fan would be attached to the train, and that would increase drag on the train. it is the loco which would have to work against all that drag, and it is by going against air that it is going to produce electricity. simple application of conservation of energy principle means that the energy produced by the Wind turbines mounted on coach would be lesser than additional energy expended by loco as the loco would have to supply power to overcome losses too.
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3)The total investment will be more if in present conditions we need to utilize solar power and also it will be not efficient one
--yes, while the cost on solar PV has drastically reduced in past few years, it is still kinda expensive as you need to buy not just the PV cells themselves, but also additional paraphernalia like convertors, which ain't cheap at all.