Powering Sustainable Transit in the U.S.

Photo: Public transit that is robust, accessible, and well-maintained will be a necessary tool for reducing traffic congestion in American cities and lowering greenhouse gas (GHG) emissions (3). All public transit options produce fewer pounds of CO2 per commuter mile than personal vehicles, with buses, light rail, and heavy rail generating a 34%, 63%, and 77% reduction in emissions per commuter mile, respectively.
Photo: Public transit that is robust, accessible, and well-maintained will be a necessary tool for reducing traffic congestion in American cities and lowering greenhouse gas (GHG) emissions (3). All public transit options produce fewer pounds of CO2 per commuter mile than personal vehicles, with buses, light rail, and heavy rail generating a 34%, 63%, and 77% reduction in emissions per commuter mile, respectively.

In the early decades of the twentieth century, Los Angeles, CA, had the largest urban rail system in the world, with over 1,000 miles of electric street car tracks spanning across four counties (1). Coverage stretched from San Fernando in the north to Newport Beach in the south, all the way to the San Gabriel Mountains in the east, making nearly the entire city accessible by rail. However, the city began tearing up the tracks and replacing them with highways in the 1950s. Today, only about a hundred miles of rail remain operational.

To those earning their living in the automotive industry, this decision, not uncommon in cities across mid-century America, was a life-sustaining infrastructural transition. To the millions of Angelenos who commute every day, this might seem like an epic tragedy. Today, Los Angeles boasts some of the most congested roads and expansive urban sprawl in the U.S. Unsurprisingly, many Americans are looking at other countries around the globe with their functional, clean, and efficient public transit with envy.

The contemporary discussion over what the U.S.’s transit infrastructure should look like comes at the same time that scientists, engineers, and governments are working to reduce greenhouse gas emissions (GHG) and mitigate climate change.

A crucial part of this transition will be cutting emissions from individual vehicles. Although electric vehicles (EVs) eliminate direct emissions, this solution is individualized, piecemeal, and contains many inherent mechanical and logistical problems, including limited capacity, long charging times, varying sustainability of the supplied electricity, and strained supply chains for crucial raw materials — not to mention the fact that they still leave you stuck in traffic. Still, even in the U.S., many cities are taking significant steps to provide sustainable and efficient transportation options.


Electric buses are one alternative for cities that requires no infrastructural renovations. More than 1,300 zero-emissions buses are currently in use around the U.S., a 112% increase since 2018 (2). With so much momentum and the 2021 Infrastructure Investment and Jobs Act allocating up to $7.5 billion for new EV charging stations, electrifying bus networks might not be a terrible idea. However, these buses use Li-ion batteries and suffer from the same issues as EVs.

A better solution could be using buses powered by hydrogen fuel cells. Hydrogen fuel cells are similar to conventional batteries in that they have an anode, a cathode, and a separating permeable membrane. At the anode, a catalyst is used to split hydrogen into electrons and positive hydrogen ions; these ions pass through the membrane into the cathode, creating a voltage difference and forcing the electrons through a circuit.

This generated electric charge powers an electric motor. The cost-effectiveness and environmental sustainability of this solution depend entirely on the source of the hydrogen. Most hydrogen used today is generated by or derived from fossil fuels.

Green hydrogen, which is produced using renewable energy, is far more expensive, as electrolyzers powered by renewable energy continue to suffer from low efficiency. Still, neither of these technologies allows buses to travel faster than the traffic around them; to solve this problem, other transit options will be necessary.

Heavy rail

Subways and trains running on elevated tracks or ground-level tracks disconnected from roads are examples of heavy rail. This style of transit allows trains to reach high speeds, avoid traffic, and transport large numbers of commuters. However, these systems are comparatively rare in the U.S. due to their high upfront and operational costs and the time and energy required for their construction.

Heavy rail is typically powered by either diesel or electricity. Although heavy rail has the lowest pounds of emitted CO2 per commuter mile of any transit option — less than a quarter than that of a private automobile (3) — room for sustainable improvement remains.

One example is the Delaware River Port Authority/Port Authority Transit Corporation’s rollout of the first solar-powered heavy rail in the U.S. (4). The line connects Philadelphia and Camden, NJ, and eliminates 53 million passenger miles worth of CO2 emissions annually.

Light rail

Streetcars and trolleys are unique as they have been powered by electricity since before the turn of the last century. Since the technology’s inauguration, one of the preferred ways to power light rail has been via a system of charge-carrying wires that run above the car called a catenary.

Depending on the system’s design, light rail operation is often unaffected by surrounding vehicle traffic, providing efficient and comfortable service. Regardless of the electricity’s source, the number of commuters that light rail can service makes it an effective GHG mitigation tool.

Light rail reduces the pounds of CO2 emitted per commuter mile by nearly two-thirds compared to personal vehicles (3). Still, the source of the electricity is one factor that affects a light rail system’s sustainability.

One American city leading the decarbonization of light rail is Seattle (5). Seattle unveiled the Link light rail system in 2009, and today it is currently powered by 100% carbon-free electricity. Puget Sound Transit, the system’s operator, sources 70% of its electricity directly from the Skookumchuk Wind Facility, while the remaining power is supplied by Seattle City Light, a carbon-neutral electricity provider.

Closing thoughts

Over the past decade, California’s highly publicized struggle to develop a high-speed rail system between Los Angeles and San Francisco has resulted in ballooning costs, logistical quagmires, and plenty of statewide frustration. Many similar stories are happening around the country, which can give the impression that any ambitious project will inevitably be condemned to languish in development.

But the wide rollout of electric buses and projects like those in New Jersey and Seattle proves this isn’t the case. For emerging technologies like green hydrogen, continued improvements will lead to innovative solutions. For proven technologies, it is up to each municipality to invest in sustainable, efficient public transportation.

  1. Hurwitz, Z., “A Look at the History and Future of Rail Transit in L.A.,” https://newsroom.ucla.edu/stories/a-look-at-the-history-and-future-of-rail-transit-in-l-a, UCLA Newsroom, Los Angeles, CA (Jan. 31, 2017).
  2. U.S. Department of Transportation, “Urban E-Mobility Toolkit,” www.transportation.gov/urban-e-mobility-toolkit, DOT, Washington, DC (June 29, 2023).
  3. U.S. Department of Transportation, “Public Transportation’s Role in Responding to Climate Change,” www.transit.dot.gov/sites/fta.dot.gov/files/docs/PublicTransportationsRoleInRespondingToClimateChange2010.pdf, DOT, Washington, DC (Jan. 2010).
  4. Delaware River Port Authority, “Powering Trains by the Sun,” www.drpa.org/projects/solar.html, DRPA, Camden, NJ (accessed on Jan. 31, 2024).
  5. Metzger, K., “Expanded Link Runs on 100 Percent Green Energy,” www.soundtransit.org/blog/platform/expanded-link-runs-100-percent-green-energy, Sound Transit, Seattle, WA (Mar. 17, 2023).

This article originally appeared in the Emerging Voices column in the March 2024 issue of CEP. Members have access online to complete issues, including a vast, searchable archive of back-issues found at www.aiche.org/cep.