The variety of currents and voltages adopted by North American railroads between 1895 and 1931 is sometimes seen as confusing. Railroads faced choices between direct current (DC) and alternating current (AC), as well as among different voltages. But closer analysis shows a clear order to the specifications chosen.
The technology options available at the time and the intended functions of these installations explain the specifications that railroads chose. Railroads electrifying fell into three basic groups: 1. Early adopters (usually requiring electrification for tunnels), 2. Railroads electrifying after 1910 for commuter service, and 3. Railroads that electrified for, or at least with freight service in mind.
Each of these faced very different sets of options, and made their choice of electrical specifications accordingly. There were four basic combinations of current and voltage: 1. Low-voltage DC delivered via third rail, 2. Medium-voltage DC, 3. Higher-voltage DC, and 4. AC (almost always single-phase at 11,000 Volts, 25 Hertz).
For railroads in a given category, the practicable choices were in fact limited to one or two options. Contrary to what is sometimes thought, the different electrical specifications reflected an underlying logic, not disorder.