The transportation system of the city of Boston comprises a combination of both rapid transit and surface lines operated under a single fare arrangement with transfer privileges permitting a continuous ride in one general direction from one end to the other of the system. The elevated lines and the Tremont St. Subway were originally built by the railway company in 1901. Today the total transportation system includes more than 500 miles of line of which 37 miles are subway and elevated tracks. The population served in the district of more than 92 square miles is considerably over a million people and the number of revenue passengers carried, approximates 350,000,000 per year. Statistics are not available as to the passengers carried on the Rapid Transit lines.