Micro-Dust Gets More Attention as Underground Rail Traffic Grows

By Dr. Bernd Hagenah

No matter the location, underground rail systems are all burdened by an invasive, environmentally hazardous problem — rail dust. Comprising mostly metallic particulates, it comes from a variety of common sources such as friction in braking mechanisms, contact between wheels and rails, pantograph contacts, and others. Unsafe levels of rail dust contribute to health problems of maintenance, operations, and cleaning staff and endanger vital electronic components of rail systems ratcheting up operations and maintenance costs.

National Air Quality Standard for Particulate Matter (PM Standard), published by the U.S. Environmental Protection Agency, defines a term “fine particulate.” Scientific studies have shown a direct correlation between the particle concentration in inhaled air and various diseases. Dust particles with a size of PM 10, having an aerodynamic diameter of 10 micrometers, could find their path deep in the lungs. In addition, particulates of this size and smaller have the ability to conduct electricity and endanger safety of electronic components including signaling and telecommunications equipment, even when such equipment is housed within dust-proof enclosures, and this can significantly reduce its reliability and lifespan.

Urban population growth and increased investment in underground rail systems suggest that rail dust has become an increasingly important factor in future rail projects. Through proper planning, however, any investments in dust minimization for existing and new subway systems could work together with scheduled improvements or new design for subway life and fire life safety systems.

Lingering Issue-in-Waiting

When the first rail tunnels were constructed some 130 years ago, air quality was seldom, if ever, discussed. Ventilation shafts were incorporated into tunnel designs solely for fire and life safety purposes, and most owners weren’t aware of the micro-dust presence or its impacts.

As of recently, this has become a more pressing concern as rail traffic and speeds have increased and environmental regulations tightened. Also, there is a growing desire for more sustainable rail designs.

There are numerous sources of metallic micro-dust; however, much of it is generated during the braking process or through friction between the power intake contacts (third rail contact, catenary system). Other basic structural conditions may exacerbate the problem including steep tunnel gradients or improper planning as noted below.

Steep Tunnel Gradients: Nowadays, steep gradients of up to 5 percent are common especially for light-rail systems. When trains consistently manage steep descending grades, the tendency of the train to accelerate increases. In this case, braking is necessary to avoid exceeding speed limits causing slippage, when large amounts of micro-dust are released.

Planning Tunnel Configuration: In single-track tunnels, the piston effect accelerates the column of air in the direction of travel, whereby the tunnel is ventilated by the rail traffic itself (so called piston effect). However, when the direction of travel alternates, as in double-track tunnels, continuous unidirectional air flow does not occur, and the column of air is accelerated in two different directions causing the stalled and often polluted air to remain in the tunnel. This phenomenon is often found in short double-track sections (portal areas) that are long enough for such air recirculation to occur; in these situations, extending center walls would provide sufficient remedy that would prevent this effect from taking place. Often, large cut-and-cover sections at portals of twin-track tunnels are areas where alternating trains push tunnel air back and forth, causing the air to recirculate and providing an “air plug” that traps the tunnel air filled with rail micro-dust within the tunnel itself. This phenomenon often results in decreased tunnel air quality and tunnel air temperatures and must be dealt with during the tunnel planning phase.

In addition, tunnel openings or not sealed cross-passages between two rail tunnel tubes may cause deterioration of the air quality. At present time, each cross-passage in the Loetschberg base tunnel has been cleaned twice a year by a crew requiring special protection and temporary fresh air supply.

Going Forward: Benefits of Early Planning

To identify solutions to the issues identified above, train induced air movements must be analyzed early in the track alignment and tunnel configuration planning process while paying particular attention to the operational concept including timetables, tunnel grades and required accelerations and decelerations, connections to ambient conditions at portal areas, and cross-passage, shaft and emergency exit locations, including connections of the tunnel with stations. The planning shall properly anticipate how the tunnel rail dust gets transported along the tunnel by the piston effect, and how the train moves the air through the tunnel while it interacts with outside air through existing ventilation systems.

Specialized simulation programs are valuable aid in the process, such as Subway Environmental Simulation (SES) or IDA Tunnel, the latter one representing a competitive software used mostly in Europe and Australia that specifically addresses air quality.

For existing tunnel systems, meeting with railway maintenance teams to pinpoint specific problem areas and concerns is of special value. Also, preventive measures might be as simple as sealing electronics cabinets or installing filters in the ventilation path to protect both people and equipment. In addition, changing the movement of the air column by separating tunnels aerodynamically might provide satisfactory outcomes. Coasting and changing rail cars from mechanical to regenerative braking might present an effective solution that also meets sustainability requirements. This could be particularly helpful in large metro systems such as New York’s, where air and wall temperatures within tunnels are a perpetual problem. A regenerative braking system creates less rail dust also while generating less heat, since the electricity is pushed back through the system to provide a power feed to other trains.

Some metro lines are considering more efficient ventilation schemes. Ideally, filtered fresh air is introduced at the beginning of a tunnel and the air is extracted at the other end before the next station is reached. In effect, the air is moved by the piston effect of the running trains. This also removes some of the braking heat trapped within the system.

In Vienna, for example, the metro line operates a ventilation system that forces as many as three air changes per hour for each running tunnel section between two stations; in some stations the supply and the exhaust air is filtered. This is vitally important, as the multi-layer underground railway network consists of five, mostly underground, routes that cross over in the stations on different levels every three to five minutes. As a result, large number of braking and accelerations occur creating large amounts of railway dust in the tunnels, stations and neighboring equipment rooms. Consequently, the tracks are cleaned during night shifts by a mobile tunnel cleaning train.

Some European lines started looking at improvements to their tunnel fan systems as well. Advances in the design and construction of more energy-efficient fan systems have come a long way and are complemented by new filtering systems that more efficiently capture dust particles in the air. The fan systems in the U.S. would likely follow the same path, but this will take time. In Europe, the energy is more expensive and urban metro lines have been forced to devise more sustainable, energy-efficient solutions that are easier to maintain.

Driverless-train systems would likely become inevitable in the years ahead and could ultimately have the greatest impact on the micro-dust problem. Metro lines in Vienna, Hamburg, and Paris have already made significant investments in driverless systems that incorporate platform screen doors at each station. In addition to contributing to lowering the air dust at stations and its impacts on passengers, the screen door systems provide for much safer rail operation as they significantly lower risks of accidents.  A driverless train can stop within a tenth of an inch of its destination, enabling the glass front to maintain a seal as the screen door opens.

Phased Approach. Regardless of the approach to minimizing micro-dust in a rail tunnel setting, periodic maintenance cycles can provide owners with an opportunity to incorporate a phased, yet sustainable solution. By doing so, they can minimize the initial financial impact and achieve gradual improvements to air quality for workers and patrons while enhancing safety and lowering maintenance costs.


Bernd Hagenah is principal engineer on HNTB’s National Tunnel and Underground Team, has a background in physics and a doctoral degree in mechanical engineering. Hagenah is based in the firm’s New York City office and serves as an accomplished resource to the firm’s clients and projects across the nation. Hagenah is internationally known as one of the world’s top technical experts and thought leaders in tunnel ventilation and aerodynamics. He previously served as senior consultant and international project manager for another tunnel consulting firm in Switzerland. HNTB’s tunnel ventilation practice has subject matter experts in rail aerodynamics, fluid dynamics and rail-facility systems operation and integration; they are located in Seattle, Oakland, and New York.

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