The train departed from Modane station at 11:15 pm.,
and descended into the valley.
The driver began the downhill descent as slowly as possible, at 10 km/h (6 mph) on the first incline, but from Freney onwards, a short distance from Modane, the train began accelerating to an eventual uncontrollable speed of 135 km/h (84 mph) as measured by the locomotive's speed indicator.
The driver applied the brakes to no effect, owing to the heavy load, and quickly lost control of the train.
Lacking sufficient braking power for the steep downgrade of 3.3%, the train derailed.

The first coach derailed while going at 102 km/h (60 mph) where the authorized speed was 40 km/h (25 mph), and its coupler broke only 1,300 metres from Saint Michel de Maurienne station shortly after crossing the metal highway bridge at Saussaz, over the river Arc.
The wooden coaches smashed into one another and promptly caught fire, triggered by the overheated brakes and lit candles which had been brought on board due to defective electric lighting.
The fire was also fed by grenades and other explosives carried without authorization by the soldiers returning home.
The fire did not burn out until the evening of the following day.
The derailment occurred at a point where the railway line passed through a narrow gap in the mountain terrain, leaving little room for heat from the fire to escape.
The train departed from Modane station at 11:15 pm., and descended into the valley. The driver began the downhill descent as slowly as possible, at 10 km/h (6 mph) on the first incline, but from Freney onwards, a short distance from Modane, the train began accelerating to an eventual uncontrollable speed of 135 km/h (84 mph) as measured by the locomotive's speed indicator. The driver applied the brakes to no effect, owing to the heavy load, and quickly lost control of the train. Lacking sufficient braking power for the steep downgrade of 3.3%, the train derailed. 
The first coach derailed while going at 102 km/h (60 mph) where the authorized speed was 40 km/h (25 mph), and its coupler broke only 1,300 metres from Saint Michel de Maurienne station shortly after crossing the metal highway bridge at Saussaz, over the river Arc. The wooden coaches smashed into one another and promptly caught fire, triggered by the overheated brakes and lit candles which had been brought on board due to defective electric lighting. The fire was also fed by grenades and other explosives carried without authorization by the soldiers returning home. The fire did not burn out until the evening of the following day. The derailment occurred at a point where the railway line passed through a narrow gap in the mountain terrain, leaving little room for heat from the fire to escape.
Saint-Michel-de-Maurienne derailment