How Subways Are Powering Cities

Every time a subway screeches to a stop, we usually chalk it up to another moment in the daily grind … if we even notice it. Commuters pour out, new riders pile in. Just another day on the rails.

But what if that screech, that everyday chaos, could do more than just move people?

In Barcelona, it does. They’ve taken the regenerative braking you might know from your EV and cranked it up to city scale. Now, their metro isn’t just moving passengers, it’s powering escalators, traffic lights … even EVs.

It’s a sneak peek into a future where energy isn’t only harvested from solar panels or massive power plants, but reclaimed from the overlooked rhythm of urban life. So here’s the question: how much energy are we letting slip through the cracks, and what happens when we start catching it?

We’re at Bellvitge Station, in beautiful Barcelona. And while it might look like just another subway stop, it’s hiding not one, but two, big secrets.

The first? Regenerative braking. That clever mechanism you might recognize from your EV, which lets us reclaim some of that wasted kinetic energy and turn it into electricity. Neat, right? But hardly new. Regenerative braking has been around since the 1800s and odds are, your car already uses it.¹ In fact, it first showed up on trolley cars, so public transit has been in on the game for over a century.

So… what’s the big deal?

Well, that brings us to Bellvitge’s second secret: what they’re doing with that electricity.²

As you probably know, trains are heavy. And stopping one? No small feat. Just ask Denzel Washington and Chris Pine. All that momentum means a lot of kinetic energy … way more than your average car can muster. And when that energy’s captured through regenerative braking? There’s enough juice to do more than just power lights at the station.

In Spain’s case, it’s being routed straight into charging EVs. Yep, you heard that right. Some subway stops are wired up to feed energy from braking trains into electrolineras (that’s Spanish for electric gas stations). The program’s called MetroCHARGE, and it’s already drawing attention from metro systems and city planners around the world.²

Which begs the question: if using regenerative braking to power train stations, EVs, and more is such a no-brainer… where has this tech been all our lives? And if it’s working this well, what else could we power with a train?

What is RB?

To understand why Transports Metropolitans de Barcelona (TMB) decided to roll with this, and how it works, let’s start at the… well, the start: regenerative braking, or RB for short.

Put simply, it’s what’s known as an “energy recovery mechanism.” It slows down a moving vehicle, or really, anything with momentum, and converts some of that kinetic or potential energy into a usable form. In most real-world cases, that means electricity.³

Technically speaking, regenerative braking systems recover energy that would otherwise be lost as heat during braking. They do this by running the electric motor in reverse, briefly turning it into a generator.⁴ As the laws of thermodynamics taught us, energy can’t be created or destroyed, only transformed. In RB, kinetic energy is converted into electrical energy, instead of just being burned off as heat.

So, how does it actually work in an EV?

1. When the driver lifts off the accelerator or taps the brake, the car’s computer tells the motor to switch into regenerative mode.
2. Instead of using electricity to spin the wheels, the motor now uses the spinning wheels to generate electricity.
3. As the wheels turn the motor, now playing generator, it produces electricity from the car’s motion and helps slow it down in the process.
4. That electricity flows back to the car’s high-voltage lithium-ion battery, giving it a bit of a top-up.

By recovering that energy, EVs improve their overall efficiency and extend their driving range. The more stop-and-go traffic a vehicle deals with, the more it benefits. That’s why RB is a perfect match for commuter trains and buses.⁵

Now, obviously, a little energy snack every time you stop is great… but RB has even more perks. The biggest? Longer brake life.

Conventional brakes slow a vehicle by physically gripping the wheels and turning all that motion into heat. This stresses out brake components like discs, calipers, and pads. Regenerative braking steps in first, doing most of the heavy lifting, while the friction brakes handle what’s left once the vehicle’s already slowing down.¹

Less heat, less wear, and fewer trips to the mechanic? That’s a win all around.

RB Development and History

But where did this tech come from and why bring it back to trains and subways?

Funny enough, regenerative braking actually started with trains. The first official RB patent was awarded to the Sprague Electric Railway & Motor Company in 1886. At the time, local electricity was in high demand and pricey, so railway owners were eager to pinch pennies however they could.

By 1908, several major tram lines in the UK were rocking the latest innovation: John S. Raworth’s “automatic regenerative control.”¹

But just a few years later, things went off the rails… literally.

In 1911, Rawtenstall tram 14, fitted with Raworth’s system, lost control on a steep hill in Accrington. It missed the passing loop and slammed into a waiting tram, injuring 20 people. The regenerative brakes were blamed, and just like that, the UK slammed the brakes on the tech for a couple of decades.⁶

A similar incident in 1948 didn’t help RB’s rep, this time in Switzerland, where a brake failure killed 21 people. Even with these early setbacks, regenerative braking was picking up speed elsewhere, making its way into trains, subways, and even funiculars.¹

And it wasn’t just on rails.

Back in the early 1900s, the first electric cars also began experimenting with regenerative braking. Models like the Baker Electric Runabout and the Owen Magnetic had early versions. Though hilariously, drivers had to manually flip switches to toggle between modes. Imagine handling that at a red light.

By 1967, tech had caught up with the times. The AMC Amitron, developed by Gulton Industries, featured fully automated regenerative braking.¹ From there … well, the rest is history.⁷

But this video isn’t about the RB system in your car. Our sights are set on something a little bigger.

So, how does TMB’s MetroCHARGE project actually work?

Barcelona and MetroCHARGE

Barcelona’s trains have used regenerative braking since the 1980s, quietly converting tons of kinetic energy into electricity. And with good reason. The Barcelona metro sees about 2.3 million riders on a typical weekday, across more than 160 stations and 78 miles of track (about 126 km).² That’s a whole lot of stops and a whole lot of energy.

All that reclaimed electricity got TMB thinking… what else could they do with it?

By the end of summer 2024, they had installed three inverters in the system, with 13 more in progress. And the results? Pretty electrifying.²

According to TMB Metro Systems Director, Jordi Picas, about 33% of the trains’ energy use is now covered by regenerative braking … enough to power 25 subway stations. And per the latest from the Associated Press, MetroCHARGE recuperators are now active at 16 stations, supplying enough electricity to meet all the energy needs of 28 subway stations.⁸

Powering the stations with the trains themselves? That’s already a smart bit of circular energy. But TMB didn’t stop there.

Some of the surplus electricity is also being routed to nearby EV charging stations. Drivers pay around 33 cents per kilowatt-hour for that train-powered top-up.² As Alvaro Luna, professor of electrical engineering at the Polytechnical University of Catalonia, told the AP:

“Since the recharging stations are installed nearby, the energy, instead of being put back into the general electric network, goes directly to the charging stations, and that allows the provider to potentially offer lower prices.”⁹

That’s a win for EV owners, and every charge helps offset the cost of installing the regenerative systems. It’s also a step … okay, more like a steady roll … toward Spain’s 2030 decarbonization goals outlined in the updated 2024 PNIEC.¹⁰¹¹

As part of that plan, all municipalities with over 50,000 residents (and island territories too) must implement sustainable urban mobility strategies through mitigation efforts.¹² Spain estimates regenerative braking will supply 41% of the energy needed to power its trains, a renewable effort expected to save roughly 3,885 metric tons of CO₂ emissions every year.² Not too shabby for something that was once wasted as heat.

Speaking of heat… that’s where things get extra interesting.

Remember, regenerative braking works by capturing kinetic energy that would otherwise turn into heat. Without RB, all that warmth builds up in the train cars and tunnels. And subways, made of concrete, tile, and earth, are great at holding onto heat. That excess heat not only makes the system uncomfortable, it wears out brake components faster and drives up maintenance costs.

Need an example? Just look at the London Underground, aka the Tube. It’s the largest metro system outside of China and Russia,¹³ and a marvel of engineering. But it’s also famously hot and muggy,¹⁴ to the point that heat warnings are common. The Central Line is notorious for exceeding 35.5°C (~96°F). Honestly, it’s not that different from Boston’s Green Line in the summer. (Spoiler: not fun.)

For context: the EU bans the transport of livestock like cows and pigs when temperatures exceed 30°C (86°F).¹⁵ If it’s too hot for pigs, it’s probably too hot for commuters.

Since implementing regenerative braking, temperatures in Barcelona’s subway system have dropped by 1.8°F (about 1°C).² Sure, that’s not massive, but every degree helps. Especially when you factor in body heat, AC systems, electronics, and climate change in general.

And climate change isn’t theoretical here. Spain has faced increasingly brutal summers, plus a string of floods and droughts that hit Valencia just last year.¹⁶¹⁷ Anything that cools things down and cuts carbon is a move in the right direction.

As the cherry on top, it’s worth remembering that subways are already one of the most efficient forms of land-based transportation.¹⁸ MetroCHARGE just turns up the efficiency another notch.

That said, no system is perfect … and MetroCHARGE hasn’t been without its challenges.

Challenges and Drawbacks

So, why aren’t we all following TMB’s lead and upgrading regenerative braking across our metro systems?

No surprise here… it’s the cost.

While many subway and train cars already use some form of regenerative braking, building out the rest of the infrastructure gets pricey fast.² EV charging stations are expensive, often unpopular, and let’s face it, cities aren’t exactly swimming in extra cash. Plus, not everyone drives an EV, and some folks don’t see charging stations as the best use of public funds. Without more long-term data, it’s hard to prove just how cost-effective a system like MetroCHARGE really is, which makes that upfront price tag a tough sell.¹⁹

There’s also a technical speed bump: not all electric rail systems are created equal. Some run on AC, others on DC. Don’t get thunderstruck, but AC systems are relatively easy to adapt for regenerative braking. DC systems? More of a problem child.

That’s because DC setups come with lossy conversions and lower voltages, making it trickier (and more expensive) to capture and repurpose energy. It’s not impossible, just costly. Dirty deeds may be dirt cheap… but going green? Not so much.⁵

Even for Barcelona, MetroCHARGE hasn’t exactly been a free ride. The project has cost about 7.8 million euros (or $8.6 million),² and we’re not talking about the biggest metro system in the world. TMB expects to earn that back in four to five years through energy savings and revenue from charging stations. Still, for the average taxpayer, that price tag can cause a bit of sticker shock.⁸

Expanding the system beyond Barcelona comes with even more roadblocks. Spain is a large, sunny country, but it has big gaps between charging points. While MetroCHARGE has helped add a few more to the network, the EV market is growing faster than the infrastructure can keep up. According to the Spanish Association of Automobile and Truck Manufacturers (ANFAC), Spain currently has around 37,000 charging points … well below the 2021 goal of 100,000.⁹

And it’s worth remembering: energy conversion isn’t magic. Regenerative braking doesn’t turn 100% of kinetic energy into electricity. Like Bart’s bicycle headlight, it’s not exactly a perfect system.²⁰²¹ Trains can recover somewhere between 15% and 45% of their braking energy. Not bad, especially compared to cars, which manage just 20%, with the rest going up in heat.⁴

Still, while it’s not perfect, it’s progress. And in a world facing rising temperatures and rising emissions, that counts for a lot.

Outlook

So where does MetroCHARGE land on NASA’s Technology Readiness Level (TRL) scale?

Well, regenerative braking itself? That’s off the charts. It’s proven tech, already commercially deployed in everything from trains to Teslas. But MetroCHARGE? It’s still climbing.

With its tech already up and running at several subway stations and electrolineras, being well-received, and expansion plans underway, I’d say it’s at least a TRL 7 (a successful prototype demonstrated in a real-world environment).²²

And it’s not going unnoticed.

MetroCHARGE has had such a strong debut that other cities are eyeing Barcelona’s blueprint. Late last year, TMB hosted a transit delegation from New Delhi, and they’re now sharing insights with officials in Vienna. They’re also bringing their findings to the Community of Metros Benchmarking Group (COMET), a global consortium of 45 transit systems that trade metro best practices.²³

Even New York City might get on board. The MTA has been exploring regenerative braking for years, and with 472 stations and 665 miles (1,070 kilometers) of track,² it’s a network ripe for energy-saving upgrades. Ahmed Mohamed, director of graduate studies in electrical engineering at City College of New York, has noted the massive potential energy savings from implementing a system like this at scale.²

Of course, as we’ve said before, upgrading metro systems isn’t cheap. Budget-strapped cities may find the upfront costs too steep, even if Barcelona expects MetroCHARGE to pay for itself in just a few years.²⁴ Still, for cities that can afford the leap, this looks like a smart way to take something already efficient and make it even better. Saving energy, money, and even cutting down on heat, all by recycling the motion of the metro.