14 March 2015

Iterative bad design

« Concerns have been raised about intermittent faults when opening the doors of the Class 377 trains at certain stations on the Thameslink route. »

The original problem? Engineers decided to use GPS location to determine whether the trains are in a station.

Unfortunately, GPS is provided by satellites, so it doesn’t work in tunnels or underground — for example, at stations like St Pancras.

The engineers’ solution: Install GPS repeaters in the stations to transmit GPS signals down the tunnels.

Except that’s still not working all the time, I’m guessing because radio signals suffer from pretty bad multipath problems in tunnels. So the solution becomes:

« The location of the train then needs to be inputted into the TMS, allowing the doors to open. In some instances, even this will not release the doors, and trains have needed to be rebooted. This can take in excess of five minutes, leaving passengers on the train without an exit route. »

Let’s enumerate what’s wrong here:

  1. Not caring that GPS is not a guaranteed service.

  2. Using GPS to trigger door unlocking, rather than a much simpler and more reliable mechanical or electronic signal.

  3. Either not realizing that GPS doesn’t work underground, or not knowing that London trains run underground.

  4. On discovering the previous error, not replacing the GPS-based system with one suitable for the task.

  5. The fact that the emergency door unlock on the train relies on the computer system working properly.

  6. The fact that the train computer takes 5 minutes to boot.

Whoever signed off on this design should lose their job.

Let’s hope terrorists don’t have the idea of combining a GPS jammer with a firebomb.

377457 Passes Horley
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