Networks of pneumatic tubes speeded mail along under city streets beginning in the 1890s. Mail was enclosed in pneumatic tube canisters that could hold up to 600 letters. The canisters traveled at an average of 35 miles per hour. The first pneumatic tubes were introduced in 1893 in Philadelphia. Boston, Brooklyn, New York, Chicago and St. Louis also used the system. By 1915, these six cities had more than 56 miles of pneumatic tubes pulsing under the streets.