In 1991–1992 the system acquired several secondhand trolleybuses from four Swiss cities. Even these vehicles, already old at the time of acquisition, became historic in their continued service after some 45–50 years or more, with 1959-built ex-Zürich number 105 being the oldest articulated trolleybus of any make still in service on any trolleybus system worldwide[13][14] for almost 20 years until its retirement in May 2015.[15] In the late 2010s, a new group of secondhand Swiss trolleybuses, 14 1989-built NAWs from the Lucerne trolleybus system, replaced all of the older Swiss vehicles, and now make up more than half of the fleet, with vintage Pullman-Standard trolleybuses continuing to make up the remainder.[16]