The more than 150-year-old London Underground is going green. Part of that transformation involves updating the materials used in its train doors, with a recent award-winning demonstration project showing that lightweight thermoplastics can replace heavy metal in doors.

Passengers likely give doors little thought as they board or exit a train. But using thermoplastics could produce a host of benefits in the “rolling stock” – engines and cars that move on the railway. “Lightweighting the doors has a cascade effect on lightweighting all of the rolling stock,” says Jeff Ive, engineering capability lead at the Bristol, U.K.-based National Composites Centre. The NCC is an independent center that works with companies and universities to develop new technologies for the design and manufacture of composite products.

Improved sustainability benefits begin with the lesser weight of the doors themselves. Then, Ive adds, lighter doors mean less closing force is required, and so the machinery to accomplish that task can be smaller and lighter. In addition, less overall weight of the subway car means that lighter brakes and smaller motors can be used.

What’s more, a lighter train means less track wear and, therefore, fewer closures for maintenance, lowering operating expenses. A lighter door also reduces the time needed to open and close it, leading to a reduction in wait time. Along with fewer closures for maintenance, this increases the number of passengers the subway system can handle by reducing the time the subway train is idle.

Even small improvements could have a significant environmental impact. The London Underground serves more than 1.2 billion passengers annually. Its subway trains log more than 47 million miles a year. Hence, environmental improvements of only a few percent could pay off substantially.

With the goal of lightweighting in mind, a consortium, which included the National Composites Centre, evaluated new door materials. In order to meet safety and regulatory requirements, any door had to overcome some challenging conditions. In particular, the material had to be strong enough to withstand rough handling. The material also had to be flame and smoke resistant, as well as meet toxicity requirements.

One composite that meets such specifications would be a thermoset phenolic resin system, Ive notes. However, a thermoplastic can offer greater resistance to impact while still offering acceptable fire resistance. A thermoplastic is better suited to high-volume manufacturing – greater than 40,000 units a year – than a thermoset, according to Ive. One reason for this is that the processing cycle time of thermoplastics is less than most thermoset polymer resins.