The Tower Subway is a tunnel beneath the River Thames in central London, between Tower Hill on the north bank of the river and Vine Lane (off Tooley Street) on the south. In 1869 a 1,340-foot-long (410 m) circular tunnel was dug through the London clay using a cast iron circular shield independently invented and built by James Henry Greathead, similar to an idea that had been patented in 1864 by Peter W. Barlow but never built.
A 2 ft 6 in (762 mm) narrow-gauge railway was laid in the tunnel and from August 1870, a cable-hauled wooden carriage conveyed passengers from one end to the other. This was not a financial success, however, and the company went bankrupt by the end of the year.