Creating a streetcar system that is predominantly solar powered is a technically feasible.  By combining old and new technology, the new Brooklyn Streetcar can be entirely powered by pollution free, renewable, solar energy.  Streetcars receive power (typically 600v DC) through an overhead wire.  Rather than exclusively utilizing conventionally generated power (from a power plant or line power),  solar panels can be used.  Solar panels, ("photo-voltaic arrays"), that converts sunlight directly into electricity, can be utilized to power a streetcar system.   24hr power can be derived from the solar power system by utilizing a battery array.  Such an array could be built at convenient remote locations. The need for any "static power converters" changing "AC" power to "DC" power for the streetcars, would be completely eliminated.  (see end section of this webpage for another power storage solution). The best place to start, is at the beginning...   About 100 years ago, the Brooklyn Rapid Transit Company devised a move-able storage battery array, to supply extra streetcar power "on demand" to certain key areas, at certain times when streetcar traffic would peak. When streetcar power demand was low, the battery array collected a "trickle charge" from the overhead trolley wire. When rail car power demand was high, the battery array could supply 600 volt power to the rail cars at the following rates: 1,000 amps for one hour, 500 amps for three hours, or 250 amps for seven hours. (Source: Street Railway Journal, June 1, 1901, pp 665- 666)   Circa 1890's, the Atlantic Avenue RR streetcar company built a power station for its new electric streetcars. This power station produced 4,400 kW (4.4 MW). This was enough electric power to simultaneously operate 100 streetcars of 60 HP each. However, those streetcars were probably only 2 axle vehicles. (Sources: The Power Stations and Distribution System of the Brooklyn Rapid Transit Company, Street Railway Journal, October 5, 1901, pp 471-480, and the The Brooklyn Daily Eagle, November 11, 1892, pg 3.) Let's now assume a 4- axle streetcar, with a 30 HP motor on each axle. This gives us 120 HP, or by using the conversion factor of 1 HP= 0.76 kW, gives us 91.2kW for maximum motoring power. Let's now add an additional 30 kW for Heating, Ventilation and Air Conditioning, as well as interior lighting. This brings us to an estimated maximum power demand of 141.2 kW per streetcar, or 235.3 amps at 600 volts DC, on level track. Let's round this off to 150 kW per streetcar, or 250 amps at 600 volts DC, maximum power demand. Since streetcars are largely "free coasting" once set into motion, this peak power demand will only occur when the streetcar is starting from a dead stop. Because the proposed streetcar line is relatively short in length, we can probably assume that only one streetcar at a time will be starting from a dead stop, and thereby requiring the full 250 amps at 600 volts, or 150 kW.   Taking streetcar "coasting" into account, this 150 kW power demand, represents the major portion of the Red Hook streetcar line's total estimated power demand, which I put at 250 kW (416.6 amps at 600 volts DC). Its assumed that at any given time, 2 of the 3 streetcars will be drawing about 30 kW each while "coasting", the power being used by HVAC, lighting, etc., while the 3rd streetcar will be simultaneously using 150kW, for starting from a dead stop.   Since streetcars spend most of their time "free coasting" on their rails, rather than wastefully, continuously, drawing motor power when in motion, 250 kW should be enough to supply ALL of the power demand for all 3 streetcars (but NOT light rail vehicles) simultaneously. Now, lets consider where the 250 kW is coming from... This power source is Solar, using photo voltaic cells to convert sunlight directly into electricity. Since photo voltaic cells are not very efficient (about 15%), a fairly large surface area directly exposed to sunlight is required, together with a storage battery array, to produce usable quantities of electric power 24 hours a day, on demand. Typically, the photo voltaic array is located on large surface area roof tops. Good examples, are Brooklyn's Nassau Brewery on Bergen Street, and IKEA on Beard Street. Photo voltaic arrays have also been successfully located above parking fields.   As a working example, the expansive flat roof of  Red Hook's Beard Street Pier, could easily provide enough surface area for a photo voltaic array producing 250kW- or rather much, much more...   If the rooftop of the Beard Street Pier were utilized, there is more than enough surface area to make the streetcar line 100% Solar Powered. Together with "regenerative brakes" used on each streetcar (converts the streetcar's braking force to electric power, which is sent back into the overhead power wire), ALL of the streetcar line's electrical power demand could be met with "clean, renewable, solar energy".   The roof of the Beard Street Pier, is roughly 700 feet x 150 ft = 11,666.66 Square Yards. The quantity of "insolation" received at the Earth's surface is typically 1 kW/ Square Meter. Since a Square Yard is 83.3% of a Square Meter, and photovoltaic cells are roughly 15% efficient, we can use the conversion formula of 0.833 kW/SY x 0.15 = 0.12495 kW/ SY x 11,666.66 SY = 1,457.749 kW, or 1.457 MW. This is enough electric power to simultaneously start over 6 streetcars from a dead stop- this translates to a medium sized streetcar system.  (Source: http://www.americanenergyindependence.com/solarenergy.aspx) Let's now look at the energy requirements for the Red Hook streetcar. Assuming our "standard constant" power demand of  250 kW (3 streetcars: 1 car starting from a dead stop, and 2 cars coasting simultaneously), then 250 kW/ 0.12495 kW/SY = 2,001 Square Yards, or 18,009 ft ², or roughly 120 ft x 150 ft of photovoltaic array, converting sunlight directly into electricity.   The 250 kW Lithium- Ion Storage Battery Arrays could be easily located at convenient places along the streetcar route.
End notes and other thoughts:
An alternative to utilizing batteries (remote power storage) is to use the power grid for power storage.  You could feed the suplus generated power (during times of most intense sun) into the municipal power grid.  Power could be converted to Ac and fed into the power grid.  Feeding power into the grid would spin the meter backwards.  During hours of darkness, power would be drawn from the grid instead of added.  As you use that power that was previously fed into the grid, the supply meter spins forward (eventually back to where it was before power was deposited into the system).  Essentially you could use the power grid as your battery. Moreover, I believe that in NY, if you produce "clean" power (from renewable energy), the power company is required to purchase it from you.