Projects in Chengdu and Moscow have confirmed that nickel battery technology, originally developed to provide onboard backup services on metro trains, can also provide a cost-effective source of power for emergency traction, eliminating the need to install separate battery packs.
Passenger wellbeing and operational resilience have long been key considerations for metro operators around the world. As such, metro networks are specified and built with a focus on day-to-day reliability. Designers and operators have sought to balance the low risk of a potential service-affecting failure against the cost of suitable mitigation measures such as building in additional redundancy.
However, it is inevitable that over time many metros will experience occasional electrical failures that affect services, potentially stranding passengers until the power can be restored. It may even be necessary to evacuate the trains on foot, although detraining passengers in tunnels or on elevated viaducts is clearly not desirable. Such outages can harm a metro’s reputation, and if passengers end up switching to cars that would result in increased road congestion, greenhouse gas emissions, and a lower quality of urban life.
So it is perhaps not surprising that in recent years some of the world’s leading urban rail operators have been investigating the potential for providing autonomous traction power, so that trains can be moved to the next station in an emergency, albeit at low speed.
Onboard battery back-up is typically provided for auxiliary functions such as lighting, door operation and communications, but there are technical challenges to using the same batteries for additional traction duties, arising from the high performance required and the limited space available on many metro trains.
Stringent safety requirements apply to any products used on passenger trains, and this is especially true for underground metros where passengers would be at increased risk from smoke and toxicity in the event of fire. Battery systems must be based on approved materials, thoroughly tested and certified before adoption.