Investigation
Initial thoughts were that the collision was 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. This valve, when open, allows compressed air generated in the first car to travel back through the other cars to power the compressed air brakes. When closed, the brakes on all cars behind the valve are disabled. Despite the signs of possible terrorism, the investigators believed that only someone with detailed knowledge of the braking system’s workings could have closed the valve.
Next, the person who pulled the emergency brake was scrutinized, though the person was unknown. When a newspaper printed an advertisement asking for the brake-puller to come forward, a young mother said she pulled the brake because she normally used train 153944’s stop at Vert de Maisones station to pick up her children from school, but was unaware of the new timetable that caused the train to skip the station. Not wanting to keep her children waiting, she pulled the emergency brake and left the train. While her actions were inappropriate, the investigators concluded, they were not suspicious.
The investigators then turned to an older interview with Saulin and looked at the record more closely than before. After the brake was pulled, Saulin went to the rear of the first car to pull the reset handle, but the handle was sluggish and refused to move. 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. When Saulin finally pulled the handle, the investigators surmise, he must have also accidentally closed the valve.
This conclusion, however, raises another question as to how Saulin got the train moving again. A fail-safe mechanism on the cars locks the brakes in the on position if the valve is closed. The only way to start the train again is to re-open the valve or manually unlock each brake pad. SNCF policy is to call for an engineer to examine the train (and who probably would have found the closed valve). However, Saulin, anxious to continue the trip, enlisted the help of Boveé to work along opposite sides of the train, unlocking each brake. Saulin believed the cause of the problem was a common condition known as an "air lock", which can occur when the emergency brake is activated. An "air lock" is when the compressed air fills up the brake pipes, clogging them and preventing the brakes from working. The brake pads lock as part of the fail-safe mechanism when an air lock occurs. To fix this problem, the trapped air must be bled out of the pipe to release the brakes.
Even with most of the brakes disabled, there were still opportunities to stop or slow the train. First, there was one more station (Le Vert de Maisons), before Gare de Lyon, which would have given the driver plenty of time to stop the train, but before reaching the station the driver was instructed to pass through and proceed directly to Gare de Lyon to make up lost time. Second, the train was equipped with an additional electric brake, but many drivers disliked the electric brake because it was unreliable. In any case, as a result of his panic the driver forgot about it.
Driver Saulin messaged a warning and activated an emergency alarm to signal that his train was in trouble, but he forgot to identify himself and his position. The emergency alarm forced all trains on the network to stop and wait for instructions, and several other drivers communicated with the controller for an explanation for the stoppage. The station staff tried to divert the train to an empty track, but the men were unable to identify which train was in distress because Saulin failed to identify himself and where he was, and since he had left his cab, he could not have responded to the controller's communication to find out which train was in distress. Then the controllers tried to message all four inbound trains to eliminate the three problem-free ones, but because Saulin had hit the alarm button, a stream of calls from many trains inquiring about the stoppage clogged communication links between the controllers and other trains, and the fact that the many messages kept the controllers busy, the controllers were never able to identify the problem train.
The signallers had pre-programmed the tracks so that the runaway train would go into Platform 1 which was empty instead of Platform 2 where the delayed train was. Crashing into an empty platform might have wrecked the train, but with the passengers evacuated into the last carriages, casualties would have been limited. But when the driver of the runaway train activated the alarm, the signallers had to turn all signals to red in a process called the "Closure procedure". But in order to give the signallers full manual control of the system, This procedure had disabled the pre-programming of tracks and allowed the runaway train to go into Platform 2 instead of Platform 1.
Read more about this topic: Gare De Lyon Rail Accident