tilting schemes such as the Talgo design which allow the train to lean into curves. The chief disadvantage is that failure of a single car disables the entire set, since individual cars cannot be readily switched in and out of the consist. Low-floor In some countries (such as the US), platform level may be below the level of the floor of passenger cars, resulting in a significant step up from platform level – leading to slower boarding times, which are important for high-capacity systems. Low-floor cars have their main passenger and loading floor directly on level with the loading platform, instead of having a step up to the passenger compartment as was traditional until around the 1970s. This is achieved by having a low-slung chassis with the "low floor" resting between the trucks, rather than resting completely on top with a simpler straight chassis design. This improved design is seen in many passenger cars today, especially double decker cars. The low floor enables easy access for bicycles, strollers, suitcases, wheelchairs and those with disabilities, which is otherwise not always convenient or even possible with the traditional passenger car design. Within the United States, the Pennsylvania Railroad developed 'The Keystone' 7 car set of cars with their own attached (head end) power car in the late 1950s. It was mostly used between New York City and Washington, DC. Boarding was at the normal platform level, at the end of the cars, with the centre section between the trucks lowered to give the cars a lower centre of gravity, and higher speed capability. Self-propelled passenger equipment These vehicles usually carry motive power in each individual unit. Trams, light rail vehicles and subways have been widely constructed in urban areas throughout the world since the late 19th century. By the year 1900, electric-powered passenger cars were ubiquitous in the developed world, but they fell into decline after World War II, especially in the U.S. By 2000 they had regained popularity and modern lines were being rebuilt where they had been torn up only 40 years earlier to make way for automobiles. On lighter-trafficked rural railways, powered diesel cars (such as the Budd Rail Diesel Car) continue to be popular. In Germany, the new Talent design shows that the diesel-powered passenger car is still a viable part of rail service. In the UK, locomotive-hauled passenger trains have largely been replaced by diesel multiple units