Three-phase AC railway electrification
Three-phase AC railway electrification was used in Italy, Switzerland and the United States in the early twentieth century. Italy was the major user, from 1901 until the 1976, although lines through two tunnels also used the system; the Simplon Tunnel in Switzerland from 1906 to 1930, and the Cascade Tunnel of the Great Northern Railway in the United States from 1909 to 1939. The first line was in Switzerland, from Burgdorf to Thun (40 km or 25 mi), since 1899.
The system provides regenerative braking with the power fed back to the system, so is particularly suitable for mountain railways (provided another locomotive on the line can use the power). The locomotives use three-phase induction motors without commutators which require less maintenance. The early Italian and Swiss systems used a low frequency (16⅔ Hz), and a relatively low voltage (3,000 or 3,600 volts) compared with later AC systems.
The overhead wiring, generally having two separate overhead lines and the rail for the third phase, was more complicated, and the low-frequency used required a separate generation or conversion and distribution system. Train speed was restricted to one to four speeds, with two or four speeds obtained by pole-changing or cascade operation or both.
The system is only used today for four mountain (funicular or rack) railways using powered carriages, where the overhead wiring is less complicated and restrictions on the speeds available less important; the Corcovado Rack Railway in Rio de Janeiro Brazil, Gornergratbahn and Jungfraubahn in Switzerland and the Petit train de la Rhune in France). They are industrial rather than low frequency (50 Hz, or 60 Hz (Brazil)), using between 725 and 3000 volts.
This category does not cover railways with a single-phase (or DC) supply which is converted to three-phase on the locomotive or power car, e.g. most railway equipment from the 1990s and earlier using solid-state converters. The Kando system of the 1930s developed by Kando and used in Hungary and Italy used rotating converters on the locomotive to convert the single-phase supply to three phases, as did the Phase-splitting system on the Norfolk and Western Railroad in the USA.
Usually the locomotives had one, two or four motors on the body chassis (not on the bogies), and did not require gearing. The induction motors are designed to run at a particular synchronous speed, and when they run above the synchronous speed downhill, power is fed back to the system. Pole changing and cascade (concantation) working was used to allow two or four different speeds, and resistances (often liquid rheostats) were required for starting. In Italy freight locomotives used plain cascade with two speeds, 25 and 50 km/h (16 and 31 mph); while express locomotives used cascade combined with pole-changing giving four speeds, 37, 50, 75 and 100 km/h (23, 31, 46 and 62 mph). With the use of 3,000 or 3,600 volts at 16⅔ (16.7) Hz, the supply could be fed directly to the motor, so not requiring an onboard transformer.
Generally the motor(s) fed a single axle with other wheels linked by connecting rods, as the induction motor is sensitive to speed variations and with non-linked motors on several axles the motors on worn wheels would do little or even no work as they would rotate faster. This motor charactership led to a mishap in the Cascade Tunnel to a GN east-bound freight train with four electric locos, two on the head and two pushing. The two pushers suddenly lost power and the train gradually slowed to a stop. But the lead unit engineer was unaware that his train had stopped, and held the controller on the power position until the usual time to transit the tunnel had elapsed. Not seeing daylight, he finally shut down the locomotive, and found that the wheels of his stationary loco had ground through two-thirds of the rail web.
Overhead wiring 
Generally two separate overhead wires are used, with the rail for the third phase, though occasionally three overhead wires are used. At junctions, crossovers and crossings the two lines must be kept apart, with a continuous supply to the locomotive, which must have two live conductors wherever it stops. Hence two collectors per overhead phase are used, but the possibility of bridging a dead section and causing a short circuit from the front collector of one phase to the back collector of the other phase must be avoided. The resistance of the rails used for the third phase or return is higher for AC than for DC because of the "skin" effect, but lower for the low frequency used than for industrial frequency. Losses are also increased, though not in the same proportion, as the impedance is largely reactive.
The locomotive needs to pick up power from two (or three) overhead conductors. Early locomotives on the Italian State Railways used a wide bow collector which covered both wires but later locomotives used two pantographs side by side. In the United States, a pair of trolley poles were used. They worked well with a maximum speed of 15 miles per hour (24 km/h). The dual conductor pantograph system is used on four mountain railways that continue to use three-phase power (Corcovado Rack Railway in Rio de Janeiro, Brazil, Jungfraubahn and Gornergratbahn in Switzerland and the Petit train de la Rhune in France).
See also 
- Railway electrification system#Polyphase alternating current systems
- Category:Three-phase AC locomotives
- Middleton p. 156
- Meares and Neale pp. 630-1 para 919
- Meares and Neale pp. 630-1 para 919
- Starr p. 347
- Middleton p. 161
- McCall p. 412
- McCall pp. 423-4
- Alternating Current Electrical Engineering by W. Tolmé MacCall (University Tutorial Press, Cambridge, 1930) pp 412–3 & 423-5
- Electrical Engineering Practice by J. W. Meares and R. E. Neale (Chapman and Hall, London, 1933) Volume III pp 542–3 (para 872) & pp 630–1 (para 919)
- Electrical Power by A. T. Starr (Pitman, London, 1937, 3rd edn 1953) p 347
- Ferrovie trifasi nel mondo, 1895-2000 by Giovanni Cornolò & Martin Gut (Parma, Ermanno Albertelli, 2000) ISBN 88-87372-10-1
- Italian Railways by P. M. Kalla-Bishop p. 98 (David and Charles, England, 1971)
- When the Steam Railways Electrified by William D. Middleton (Kalmbach Books, Milwaukee, 1974) ISBN 0-89024-028-0