Tuesday, December 3, 2013

Metro-North crash and the push-pull argument

The tragic crash of the New York Metro-North train, which derailed coming into a sharp curve last weekend and killed 4 passengers, is likely to renew concern about the push-pull system.  The train was cab-car forward, with the engine at the rear.  It looked from the photographs like a Gennie, which is the GE-Genesis model used by Amtrak on its long-distance lines (and different from our usual Pacsurfliner F59s).

In discussions with non-train-riding friends on line, I had to explain that no, the engineer is not in the locomotive facing backwards when it is pushing.  People find it scary to think there's no one in the locomotive, but I reminded them that when they see a big freight with 4 engines, the followers aren't staffed either.

I also had to explain that trains live in a one-dimensional world, and can't simply turn around.  Instead, they must find a wye and jockey back and forth to reverse direction.  This isn't very practical for most commuter runs, a complication compounded by "stub" stations like LA Union, which dead-end in one direction.

Why did the Metro North train enter a sharp 30mph curve at 82mph?  The news today suggests that the engineer had dozed off.  I thought the throttle used a "dead man's hand" system, where a loss of tension would slow it down;  if it does, the engineer didn't loosen his grip while he dozed.  By all accounts he was a solid engineer.  It's a tragedy however it happened, and our thoughts are with the injured and mourning.


Anonymous said...

Most modern trains use what's called an "Alertor" system. The Alertor system works such a way that the computer monitors engineer control inputs. If it detects a period of inactivity while the train is moving (The length of which is determined by train speed) it flashes a set of lights. If the engineer doesn't respond by manipulating the controls within about ten seconds it sounds an alarm, and if still no response within the next 10 seconds or so the computer initiates a "penalty stop" by cutting power from the engine and gradually letting all the air out of the brake line.

The cab car in this wreck was older, and did not have the Alertor, but it *Did* have the "dead man" function. The "Dead Man" pedal is a pedal with a lot of resistance which pneumatically connects to the PCS and brake line. By taking your foot off of it (or, in theory, passing out/dying and losing the muscle tone required to hold it down) it immediately initiates a penalty stop. The problems with Dead-man pedals which led to the development of alertors is that it's easy to leave a heavy backpack on the pedal, thus defeating it, and in cases such as this accident the engineer wasn't exactly awake nor was he asleep, so he was holding the pedal down subconsciously.

Officially Metro-North has announced that in response to this accident they'll be replacing the remaining Dead-man pedals with Alertors. They also are updating cab signals (which work in concert with the onboard computer to force trains to comply with signals by initiating a penalty stop if the train doesn't appropriately respond to them) to enforce major speed reductions such as this 30 curve by programming the signal system to act as though they are restrictive signals.

None of the railroads on the West Coast use cab signals, although Metrolink uses IIATS which performs a similar function. Difference is it doesn't enforce speeds, it only sounds an alarm and requires the engineer to push a button in acknowledgment of upcoming speed reductions. Once he acknowledges it he can go as fast as he wants and the computer won't enforce the speed change. An IIATS system might have worked in New York though, as the alarm may have woken the MN engineer out of his daze with enough time to slow the train.

Metrolink installed the IIATS system as a fast and cheap solution after an incident which was largely kept from public knowledge in which a student engineer operated a train through a 45 MPH curve at about 75 MPH. Fortunately no derailment or injuries occurred except to the careers of the student and the manager who was supervising him.

IT said...

As always, very helpful and informative, Anon.