In Edison's day, there was not a realistic way to step-up and step-down the DC voltage as economically as transformers.
In Modern times, however, we do use some DC transmission lines where large amounts of power are transmitted long distances with few (if any) intermediate taps.
Typically these are on the scale of 500VDC. We have transmission lines like this in the US and have done so purely as an economic transmission system and not related to solar or wind power.
HV Direct Current Transmission System
http://www.energy.siemens.com/hq/en/power-transmission/hvdc/
The DC line cost more at the terminals where power is stepped up or stepped down using power electronics. But the Transmission line cost less money and has less line losses. If you go a long enough distance with only power taps at the beginning or end, DC will cost less over a long distance (hundreds of miles).
The towers are easy to identify as they don't have three phases.
For example, the Pacific DC Intertie is a 500kV DC line bringing power from the Northwest hydro-generation to Los Angeles.
This line was first energized back in 1970 and was later increased from 400 to 500 kV. It is now rated for 3,100 MW of transmission.
Pacific HVDC Intertie
http://www.abb.com/industries/ap/db0003db004333/95f257d2f5497e66c125774b0028f167.aspx