Early train telegraph patents

The earliest patents for train telegraphs were based on establishing a continuous electrical connection between the telegraph line and the moving train, by means of a rolling or sliding contact. An example is James G. Smith's US Patent 141604, issued 5 August 1873.

Later plans, such as William Wiley Smith's US Patent 247127 (issued 13 Sept 1881), dispensed with the physical contact between train and telegraph wire, instead relying on capacitive or inductive coupling (via electric or magnetic fields) between the train circuit and the telegraph line. Thus the signals would pass between train and line through the intervening space.