Exclusive, completely separate subway tunnels, tracks and related station and access facilities make subways among the safest transportation modes and services. With the exception of derailments, collisions or failures of subway equipment itself, there exist virtually no other threats or hazards to subway operators or passengers. Incidental accidents or failures generally result from nonsystemic causes, which can be mitigated and prevented without redesign or fundamental change to subway equipment or systems.
Most hazards and destructive impacts of subway development occur during construction, and range from the hazards and dangers of tunnel drilling, station excavation and construction, and their secondary impacts on surrounding properties, building foundations, utility and infrastructure systems and street networks. Schools, hospitals, libraries and other institutional settings may be temporarily disrupted during subway construction, but are seldom affected by operations of completed systems.
The confined and isolated nature of subway tunnels and underground stations, and their confined passengers, increase subways’ vulnerability to criminal and terrorist attacks, while power failures or fires of any type pose significant hazards to anyone trapped below ground, under any such conditions.
While the 2010 complete electrical failure and shutdown of ice-encrusted, frozen trans-English Channel passenger trains some one to two miles into their under-sea tunnels, trapping passengers in frigid, pitch darkness for many hours, was certainly a harrowing and dangerous experience, the deadly dangers created by the July 2011 flooding and overflowing of the Han River into the streets, freeways, open spaces, buildings and subway stations of Seoul, South Korea, posed great danger to virtually all passengers, operators and subway personnel who may have been trapped by station or tunnel flooding, as well as, any occupants of subway trains shut down by electrical failures or flooded sections of subway system tunnels. Although the trains stalled in the channel tunnel could be eventually thawed, dried, and put back in service in relatively short time, the flooded and submerged Seoul subways could require weeks or months of pumping and repair of facilities and equipment to be safely put back into service, with little or no guarantee that flooding will not recur in the future.