Some Trains Have Rubber Tires Like Giant Buses And The Reason Why Makes Total Sense

Most of the trains most people see every day roll on metal wheels guided by metal rails. Everything from majestic freight trains and regional commuter rail to New York City’s iconic subways are like this. Yet, sometimes you’ll find a different kind of passenger train, one that has pneumatic tires and rides on concrete rather than real rails. They look like guided buses, but why do they exist and why are you going to find a lot of them at airports? Once you learn about these trains you’ll see they make a lot of sense.

Our fearless leaders have been traveling across Europe and one thing has caught David Tracy’s eye. Deep in the Paris Métro, he spotted rolling stock known as MP 73 and MP 89 CC. Apparently, those alphanumeric names mean something and MP 73 is Métro Pneu appel d’offres de 1973, or a rubber-tire metro ordered in 1973. The MP 89 is the same thing but was ordered in 1989. But as their name implies, these aren’t any normal metro trains. As David observed, these things are rolling down flat surfaces using inflated rubber tires!

You don’t have to go to Paris to find other trains like these. Rubber-tire metro trains can be found in the Montreal Metro as well as so many airports from Germany to Taiwan and at nine additional airports here in the United States. If you’ve been to Denver International Airport or the Phoenix Sky Harbor International Airport, you’ve probably taken a ride on one of these trains. But why?

From The Dawn Of The Pneumatic Tire

As the National Museums Scotland writes, Scotsman Robert William Thomson invented the pneumatic tire and patented it in 1846 after utilizing the then-new process of vulcanization to produce rubber that was pliable, but not sticky. John Boyd Dunlop would later invent his own pneumatic tire in 1888, turning it into a revolutionary product for bicycles and later cars.

According to the book History Of The Pneumatic Tyre, Thomson saw his tire, named the Aerial Wheel, as being a perfect solution for horse-drawn carriages but also for rail. Thomson was a railway engineer and immediately saw advantages in having carriages ride on rubber. For horse-drawn carriages, Thomson felt his Aerial Wheel would lessen the power required to move carriages while also cutting down on noise. When used on the rails, the Aerial Wheel reduced noise and increased ride comfort.

Thomson’s vision for a railway version of the Aerial Wheel saw a train running on Aerial Wheels, which rolled down a board track. Steel wheels stuck around, but they were flipped on their sides and rolled down a central rail to guide the train down the tracks. Spanish engineer Alejandro Goicoechea, the founder of train manufacturer Talgo, patented his own version of the rubber-tire train in 1936.

Despite these early efforts, it was France and Michelin to make the rubber-tire metro train mainstream, from Michelin:

In the 1930s, the RATP (Régie Autonome des Transports Parisien) raised the question of equipping its subway trains with tires to improve transport conditions and facilitate the operation of these lines. It then naturally turned to the railcar imagined by Michelin who had just broken the record of the Paris-Deauville link in January 1931: the famous Micheline. Our collaboration with the RATP ended twenty years later, in 1951, with the circulation of the first metro on tires in the world, on an experimental line intended to test the innovations developed by the RATP: line 11 (Châtelet – Mairie des Lilas).

The benefits of this technical solution are quickly apparent: acceleration and braking are improved, which increases the speed on the lines offering short inter-stations. The trains run better on the slopes and are better at cornering, limiting the constraints. Tires also reduce vibrations and noise, for better comfort for travelers and residents! Vibration reduction also extends the life of the equipment, whether it is the rails or the bogies that make up the train.

So many qualities that will make the success of the metro on tires in France, then in the world. The same technology is adopted for rail shuttles serving most airports and is well suited for automated lines, which have been booming in recent years. When a public transport operator launches a “pneumatic” line, it commits itself for several decades. He therefore needs a reliable partner. Our 70 years of experience is a real asset, which explains why Michelin equips 8 out of 10 trains in the world.

Still Relevant Today

That middle paragraph is the important bit. Have you noticed how quickly airport metro trains can get you around?

As Michelin explains up there, trains running on rubber tires can get moving and stop quickly thanks to the much higher friction of rubber against metal or concrete compared lines to smooth steel wheels against smooth steel tracks. This helps to increase line speeds between stations. The tires also add track flexibility as they climb grades like cars do. I’ve seen airport systems climb hills and take the kinds of sharp turns a traditional subway train wouldn’t be so happy with.

As Railway News, the publication of French equipment manufacturer Texelis writes, the first generation of rubber-tire metro trains was the MP 55, which was built in a collaboration between Renault, Brissonneau et Lotz, and Alstom. In keeping with the theme I told you about early on, these trains were rubber-tire metro trains ordered in 1955 with deliveries beginning in 1956. One of the big advantages offered by the MP 55 was its ability to achieve higher speeds than the previous Sprague-Thomson steel wheel trains. A Sprague-Thomson achieved speeds of just 28 mph while the trains on rubber tires hit up to about 43 mph.

The French version of the electric rubber-tire metro uses both flat platforms for the tires to run on but also steel wheels and tracks directly next to the rubber tires. Here’s an example of a bogie for the MP 89 series:

Railway News reports that trains like these are still relevant today. These trains’ abilities to stop and go quickly, combined with modern signaling and control systems, allow a crowded metro like the one in Paris to operate trains with high frequency and closer together. Passengers also enjoy reduced noise since they’re riding on inflated tires rather than metal wheels.

Railway News continues its list of benefits by pointing out a rubber-tire train’s excellent climbing abilities but also reduced vibrations against the bogies, allowing a train manufacturer like Texelis to use cheaper, lighter materials. Finally, Texelis believes the rubber tire system is cheaper to maintain. Keeping the steel wheels around also offers redundancy in case a rubber tire blows.

Why Aren’t These Everywhere?

So, if rubber tires are so great for trains, why aren’t the primary form of passenger rail? You’ll also find trains running on rubber tires when you ride a Disney Monorail. Still, what gives?

A technical paper written by Matthew C. Harrison and published to SAE International gives us a good explanation. Let’s start with the downsides you’re probably already thinking about. Trains with rubber tires may not be proficient at their job when the path ahead is covered in ice. Tires can also blow out, which doesn’t happen with steel wheels.

Then the SAE paper digs deeper. Trains running on rubber tires require more energy to move than their steel-wheel counterparts. There’s also the higher maintenance of needing to replace and servicing several tires. You’ve probably also noticed another problem and it’s the fact that these trains still use steel rails and wheels for guidance and redundancy, so railways don’t save money when it comes to trackage. To put this into perspective, one student paper from McGill University found that the Paris rubber tire metro runs 2,688,000 miles per $1 million in maintenance, but Toronto is able to go twice the distance for the same money on steel wheels.

(Correction: The original version of this story mixed Toronto with Montreal. Toronto is the one with steel wheels.)

Other noted downsides include the fact tires break down into particles which add to pollution and that while rubber-tire trains are smooth, steel wheel trains running on well-maintained tracks are still smoother. Our own Adrian Clarke also points out that tires aren’t great when you’re dealing with really high loads or intensity. Apparently, all the heat generated by the tires is also a problem. Putting all of that heat energy into underground tunnels means some of these trains cannot have air-conditioners, because AC would add even more heat into the mix.

These trains are relevant today, but they’re best for specific use cases like carting people around airports or providing lots of train intervals in a metro system. Otherwise, old-school steel wheels and rails still win out for operations.

So, the next time you’re at an airport or one of the many cities in the world with metro trains running on rubber tires, you now know why. You’re going to be whisked away to the next station quickly and quietly thanks to the same tech that allows your car to get you around.

(Images: David Tracy, unless otherwise noted.)