The city of Sheffield, U.K. is installing the country's first tram-train, a public transit hybrid that can serve both the suburbs and a city's urban core thanks to a special wheel design that can handle both the tight turns and higher speeds.

When it's complete in 2016, tram-trains will run eight miles along the national rail tracks from Rotherham Parkgate to Sheffield, just like a traditional commuter rail line. But instead of stopping at a station at the edge of the city and forcing riders to switch to local transit, when the train gets to Sheffield's center – er, centre – it will seamlessly transition onto the existing rails of the Sheffield SuperTram.

Sheffield's system is a $98 million pilot project to determine whether tram-trains will work in the rest of the U.K. On paper, at least, it seems like a good fit: The tram-train setup not only gives passengers an easier ride downtown, but turns any existing rail right-of-way into a potential light rail system. Maintenance costs are lower for trams, and a tram-train gives transit authorities added flexibility.

Technologically speaking, building a tram-train may not sound like a spectacular feat, but trust us, it is. Even if they're of the same gauge of track, the two modes often have different signaling and safety standards, and a tram-train must comply with both.

More importantly, train wheels run on top of raised rails and rarely have to make tight corners. Tram tracks are often flush with street surfaces, so the wheels are designed to ride inside the grooves. And trams are also expected to make hairpin turns. Put the wrong kind of wheels on the wrong kind of track, and you'll derail, especially at junctions or corners.

A special wheel design that takes into account both setups is a key part of the Sheffield tram-train. Developed at the University of Huddersfield's Institute of Railway Research, it gives special consideration to worn rails and wheels.

The tram-train may be new to the U.K., but it's been a well-known part of the public transit network in Karlsruhe, Germany since 1992. In fact, the very idea of running trains and trams on the same tracks is known as the "Karlsruhe model."

Since then, other tram-train systems have sprung up, mostly in Germany. In North America, New Jersey's River Line and Ottawa's O-Train are two examples, but they haven't quite caught on in the U.S.