engine, and the longitudinal stability of the vehicle is maintained by active hydraulics mounted under the turntable. This modern system makes it possible to build buses without steps and having low floors along their entire length, which simplifies access for passengers with limited mobility. Modern low-floor pusher articulated buses also tend to suffer from suspension problems because their wheels lack sufficient travel to enable them to absorb typical road surface unevenness. This also leads to passenger discomfort and relatively rapid disintegration of the vehicle's superstructure. Makers of pusher-type articulated buses include Mercedes-Benz, New Flyer Industries, MAN, Volvo and Scania. The Renault PR 180 and PR 180.2 (articulated versions of the PR 100 and PR 100.2) were a special variation of the pusher design in which both the middle and the rear axles were driven, with a driveshaft passing through the turntable between the two driving axles. Alternative power Although the majority of articulated buses utilise diesel engines for their motive power, a number of operators (primarily outside North America and by LACMTA) have adopted compressed natural gas (CNG) power in order to reduce pollution. Many other transit authorities in the United States and Canada are adopting articulated buses that are diesel-electric hybrids, such as the New Flyer DE60LF. There are also articulated trolleybuses, which use catenary cables to power electric traction motors. Electric articulated trolleybuses principally operate in hilly locations like Mexico City, San Francisco, Seattle, and Vancouver, B.C., where the steep grades preclude the use of combustion engines for motive power. The New Flyer Xcelsior Charge NG battery-electric articulated buses are equipped with traction motors on both the middle and rear axles; the middle axle uses in-wheel motors. Articulation Super-articulated buses Super articulated bus are similar to the normal articulated buses, but they have a fourth tag axle next to the rear axle, increasing the rated load. Typical capacity ranges up to 200 passengers and the length increases from . Examples include the Mercedes-Benz Citaro CapaCity, MAN Lion's City GXL (A43), Mercedes-Benz O500 DA, BYD D11, and Scania K IA. Bi-articulated buses Since the late 1980s, the concept of the articulated bus has been extended further with the addition of a second trailer section that extends the bus almost to tram length and capacity, to create a bi-articulated bus, also called a triple bus. The Chinese manufacturer Zhejiang Youngman (Jinhua Neoplan) has developed the JNP6280G bi-articulated bus, deemed the