A recent metro can also lose traction power and continue to power all third of the lights in the tunnels and rail cars with two hours still to go, a design option that was intended to turn an absolute worst-case situation into an orderly walkout and not into a blind scramble.
The fact that metro tunnels are created to move rather than stay still turns the tunnels into dangerous areas within the course of seconds. The narrow clearances and few access points that enhance efficiency of underground rail also squeeze when power goes away which are the same properties that limit access. The symptom of darkness is mere. In the background it starts all interacting, ventilation, communications, train control, passenger decision-making, and all these start to interact in a manner that can escalate small failures into system level issues.
The injury of traction power does not imply that a train is immediately brought to a halt, and it is that distance between “no power” and “at rest” that makes tunnels intolerant. In common electric and diesel-electric systems, traction motors are still mechanically attached to the wheels with fixed gearing such that when braking is released the vehicle can enter coast instead of stalling. Although this is technically a discussion on rail traction, it says that nearly all trains, whether electric or diesel-electric, have motors linked directly to the wheels by a fixed gear ratio. The act of braking is then a controlled combination of the means available; regenerative braking when the electrical system is capable of receiving the energy, and pneumatic friction brakes when not. A regenerative braking system can be constrained by under-voltage cutoffs on a de-energized line, or by having no place to emit recovered power to, placing an extra load on air brakes and onboard reservoirs intended to sustain numerous complete braking cycles. After a train halts between stations, the tunnel in itself has become the main safety system.
At that time, lighting, communications and wayfinding do not become a luxury, they become the scaffolding against disorientation and crowding. One historic design to “provide evacuation, rather than comfort,” is the Metro instruction of Washington, D.C., on emergency back-up batteries designed to continue to provide partial tunnel and car lighting, and longer station emergency lighting, during an emergency. According to the same release, “Metro patrons will not be trapped in Metrorail tunnels in the dark” and the assertion is supported by backup power, staff on the trains, and printed procedures. Technically speaking, that is a stratified mitigation approach: keep the lighting levels to a minimum, keep the public-address line, and keep the operator-to-control-center communications sufficient to shift the “mode of operation” to “people movement.”
That transition is made difficult by the confined geometry. Advice on tunnel hazards states that equipment failures like faulty lighting or ventilation should emphasize that what may have started as a nuisance can be dangerously turned, and the enclosure of tunnels enhances the effects of malfunctions and blaze. The same source explains that poor visibility can entrap people before the responders can arrive, as it is also characterized by the occurrence of smoke, which is very opaque, incapacitating and also significantly hinders the evacuation of users. Reduced visibility in the narrow egress paths brought on by absence of fire increases the chances of trips, falls, and bottlenecks even in situations where the passengers have moved off high platform edge and onto the walkways to be used by maintenance personnel and not to evacuate mass of people.
Operators hence react to power loss less as an electrical failure and more as a human factor incident with a time bomb. The initial minutes are characterized by behavior: do passengers remain, self-evacuate or seek to forcibly open doors. The tunnel-risk guidance reminds the fact that “the only people in the tunnel” are the users until emergency services are reached and therefore responsiveness and compliance is crucial. In Metro systems, drills, training, and uniform signals attempt to influence such behavior as one of the worst failures in a dark and closed tube is not necessarily the one that began the failure, but the cascade of failures that occur when no one can give directions at the moment the situation most requires it.
