Boston's subway system uses 600 volt DC for third rail and overhead wire powered electrical trains. Washington DC's metrorail system uses750 volt DC on it's third rail. Boston buys regualar 60 Hz power, and converts it to DC at various substations around the system.
As an amusing sidebar, read about the marketing wars between Westinghouse and Edison when they were battling over DC versus AC power as the standard. One of the sort of sad ones were that both of them made electric chairs to be used for executions. Westinghouse's chair used DC current, because, he claimed, it was more deadly. I believe Edison's chair used AC current, for the same stated reason.
--Dale[TELECOM Digest Editor's Note: The last hanging (as a method of execution or capital punishment) in Chicago occurred in 1921. They used to erect the gallows needed for the occassion in the alley on Hubbard Street behind Clark Street at the old Cook County Courthouse. That same year -- 1921 -- the new courthouse and associated jail were constructed on the (then booming and showing promise) southwest side of the city, 26th and California Avenue. They proudly proclaimed that henceforth they would be using the 'much more humane' method of execu- tion devised by Thomas Edison, an 'Electric Chair' as it was to be known, and they gave a tour of it in the basement of the (then) newly constructed county jail. On the first occassion of its use in 1922, Mr. Edison was a guest of honor, giving a short speech about the workings of the Chair before a crowd of civic leaders a few minutes before it was put to use. His Chair was used for more than forty years, with the last execution taking place, I think, in 1965 when executions were transferred from Chicago to Joliet, IL at the prison there. PAT]