Wednesday, 4 April 2018
Sometimes, it is worthwhile to re-examine your standards. Is your use of standards too rigid? Are you locking yourself into over engineering your railway? Are you sticking to the rules because they are the rules? Are there opportunities to gain time, reduce costs, improve visual impact or upgrade the passenger experience by adjusting standards, re-analysing risk or getting a derogation? This article offers a small but important example of how a standard could be modified to good effect.
This case study on the rigid use of standards is on train door operation. If we assume door opening is activated by a member of the train crew, we would want to ensure that the doors remain closed until the train has stopped. Once wheel stop (zero speed confirmed) has been detected, the on-board system can unlock the door opening controller and the crew member can release or open the doors.
Fine? Yes, as far as it goes. It is definitely safe but this arrangement causes a considerable lag between wheel stop and door opening. Time is needed for wheel stop to be detected, for a release indication in the cab to appear, for the train crew to operate the release buttons, for the door controllers to receive the signal and for the doors to begin opening. On many main line trains, it can be 7-8 seconds before the doors are opened for passengers. On a metro or suburban service, it can be 3-4 seconds. An extra four seconds at every station on a line like the London Overground route between West Croydon and Highbury & Islington adds 76 seconds to the train’s journey time. This is half a train path and buying a peak-hour train path is expensive, so both time and money are being wasted. Perhaps there's a better approach? Perhaps we could use human experience to inform automated system protection?
In the days of guard door operation on the London Underground (finally abolished in 2000), there was no interlocking of doors with train movement. Doors could be opened while the train was moving. They weren’t of course (usually) but the facility was used to speed up station operation. The experienced guard knew that there was always going to be a lag between the train wheels stopping and the doors opening fully and he knew also that station stop time had to be kept to a minimum. To keep dwell time short, the guard would press the door open buttons a second or two before the train stopped so that the doors were beginning to open at wheel stop. Valuable seconds were saved and no one got hurt. So, could this be replicated in an automated system?
Of course it could. It fact, it already has. In January 1980, the first D Stock train went into service on the District line with a new door control system that kept the train door control locked while the train was moving but which allowed the guard to release the doors if train speed was detected below 4 mph. It replicated the human experience that allowed time to be saved at stations. It remained in use for many years but the idea was lost in the safety panic that ensued after the Kings Cross fire of 1987. Perhaps, now, with efficient train throughput at the top of the railway agenda, is the time to review the rigid zero speed restriction on door opening and make better use of the technology available for the digital railway. And, are there other examples where this approach could be beneficial?