As the train neared Paris, it passed the Le Vert de Maisons station, which was usually a stop on the route.

Driver Daniel Saulin, assisted by Guard Jean Charles Bovée, after working for 26 minutes, reset the brakes and continued.

To make up the lost time, the station controller at Gare de Lyon, André Tollance, instructed Saulin to skip the next scheduled stop (and the last before the terminal), Maisons-Alfort–Alfortville.

As the train picked up speed from its descent, Saulin desperately radioed an emergency warning, but failed to identify himself to the controller.

Initial thoughts were that the collision was the result of a terrorist act, due to a recent string of bombings of French trains in and around Paris.

This theory was supported by the fact that investigators who were examining the wrecked shell of train 153944 found the main brake pipe valve, located behind the first car, closed.

She told investigators that she normally used train 153944's stop at the Le Vert de Maisons station to pick up her children from school.

Having been overlooked previously, it was discovered that Saulin had placed his hand on the main brake pipe valve to get more leverage to pull the reset handle.

However, Saulin, anxious to continue the trip, enlisted the help of Bovée to work along opposite sides of the train unlocking each brake.

Nevertheless, Saulin had no idea of what he had done, and stated that the pressure gauge in his cab was marking the correct measure, which is why he thought all the braking systems were in working order.

Driver Saulin messaged a warning and activated an emergency alarm to signal that his train was in trouble, but he neglected to identify himself or his position to the controllers, and left his cab before anyone could call him back.

A resulting stream of calls from waylaid drivers inquiring about the stoppage clogged the communication lines between the control room and the trains.

Legal proceedings were begun against the Gare de Lyon supervisor, Auguste-Andre Tholence, who failed to evacuate the station, and Mirroir, but were unsuccessful.