The electrification

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Early in 1902, the line reached its maximum capacity. Even with the increase of the rolling stock roster, it was not possible to schedule more than 17 daily round trips (between 4am and 9pm). It became mandatory to increase the global pulling power of the locomotive roster. Only electric technology could offer the 500 HP needed to increase revenue.

In 1903, electric traction was tested on a section of the line between La Motte-les-Bains and La Motte d'Aveillans (6.635 km). The test where conducted by a swiss engineer: René THURY, director of the "établissements SECHERON", later to become ABB-SECHERON. Looking back to that time, THURY's ideas in that domain can undoubtly be called "revolutionnary", with the evidence of their pertinence and efficiency.

All electric railroads at that time had preferred 600 Volts DC. THURY claimed 2400 Volts DC would be the solution. The 2400 Volts where fed thanks to 3 wires: 2 contact wires, respectively +1200 Volts and -1200 Volts and the rails at 0 Volts. This technic a.k.a. "two bridges" tended to better traction yield; it limited the voltage applied to the traction motors.

The SGLM hence became the first railway line to be powered by high voltage DC traction. The energy was provided by the hydroelectric power plant of Avignonet, built over the Drac between 1899 and 1902.
Two rotary converters were installed at the Avignonet plant and DC voltage was transported to the La Motte-les-Bains substation. Later, the converters were installed in the substation, and the electricity between the plant and the sub-station became AC, 50 Hz.

The contact wires, of 113,1 mm² cross section, where kept parallel 1.4 meter away by non conductive stanchions. Wooden poles kept the cob web 4.3 to 5 meters above the rails (4.15 in tunnels). Switch design became a nightmare: how to make two +1200 and -1200 DC Volts line cross eachother !

An important number of battery cells where set in 1911, at La Motte-les-Bains, to limitate contact line potential variations. As complement, both ends of the line where equiped with a set of rotary machine able to compensate over or undervoltage variations. The rotary converters equiped La Motte-les-Bains sub-station was assisted in 1932 by a second sub-station fitted with mercury arc rectifiers. It was located above the Ripeaux tunnel.

Poles where set every 35 meters on straight track and 26 meters on curves. An upgrade job started in 1905 where concrete masts replaced the wooden ones. Today, most of these masts are still the same, even if the rust of the iron rods cracked the concrete. Undoubtly, La Mure Railway catenary is today the world oldest, and still in active service !

Since the 1950's, both contact wires have been joined, and carrying now 2400 Volt DC. The SGLM, though, is one of the last to operate under this voltage, as well as the Rethic Railways on the COIRE-AROSA section.

In 1985, the Ripeaux sub-station is removed from service. At the same time, the La Motte-les-Bains sub-station is upgraded with silicon rectifiers. This sub-station is now a real museum because the old equipment has not been removed. There can be seen: rotary converters, mercury arc rectifiers and solid state rectifiers (not open to public).

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Rail Modélisme Sud Isère

	All electric railroads at that time had preferred 600 Volts DC. THURY claimed 2400 Volts DC would be the solution. The 2400 Volts where fed thanks to 3 wires: 2 contact wires, respectively +1200 Volts and -1200 Volts and the rails at 0 Volts. This technic a.k.a. "two bridges" tended to better traction yield; it limited the voltage applied to the traction motors.
	The SGLM hence became the first railway line to be powered by high voltage DC traction. The energy was provided by the hydroelectric power plant of Avignonet, built over the Drac between 1899 and 1902. Two rotary converters were installed at the Avignonet plant and DC voltage was transported to the La Motte-les-Bains substation. Later, the converters were installed in the substation, and the electricity between the plant and the sub-station became AC, 50 Hz.
	The contact wires, of 113,1 mm² cross section, where kept parallel 1.4 meter away by non conductive stanchions. Wooden poles kept the cob web 4.3 to 5 meters above the rails (4.15 in tunnels). Switch design became a nightmare: how to make two +1200 and -1200 DC Volts line cross eachother !
	An important number of battery cells where set in 1911, at La Motte-les-Bains, to limitate contact line potential variations. As complement, both ends of the line where equiped with a set of rotary machine able to compensate over or undervoltage variations. The rotary converters equiped La Motte-les-Bains sub-station was assisted in 1932 by a second sub-station fitted with mercury arc rectifiers. It was located above the Ripeaux tunnel.