(2018), which are all wheelchair-accessible (low floor) and around 55% are air-conditioned. The fleet utilisation rate during workdays stood at 78.1% in 2018. Between 2005 and 2009, the fleet underwent its most substantial renewal since 1989, and is now among the most modern in Europe. In December 2005, STB signed a contract for 500 Mercedes Citaro low-floor, wheelchair-accessible buses. The buses were delivered between June 2006 and April 2007. In January 2008, the first of a further series of 500 Citaro low-floor buses were put into service. The fleet makeup, as of 2019, is as follows, with years of delivery shown in brackets: Central Lines are usually populated by Otokar and Hybrid buses, while lines connecting rezidential areas tend to use the older Citaro models. Trolleybuses Trolleybuses supplement buses on the STB system network, which operates 15 trolleybus lines (mainly on high-usage routes), measuring of routes on network. During the 1990s, the fleet was updated with modern trolleybuses manufactured by Ikarus, in light blue and yellow livery, which have acoustic station announcements and digital display screens. These trolleybuses now make up the majority of the fleet. In early 2007, 100 wheelchair-accessible Irisbus Citelis trolleybuses were introduced on routes 61, 62, 69, 70, 86, 90, 91, and 92. Two distinct, non-interconnected networks exist in Bucharest, the main network (comprising the two main east–west lines as well as a spur in the northern part of town) and a relatively small southern network in Berceni. Each of the networks has its own, separate trolleybus depots (including separate fleets) and is not connected to the other in any way, in 1987 being separated after the opening of M2 metro line. The trolleybus network is currently being overhauled or expanded — a goal is to eventually unite the two networks. Fleet In 2018, the STB had a fleet of 265 trolleybuses spread across 3 depots and 1 mixed tram-trolleybus depot. Most of them were Astra-Ikarus 415T (163 trolleybuses). The fleet utilisation rate during workdays stood at 61.8% in 2018. Trams and light rail STB operates a complex system of trams measuring of routes on of lines throughout Bucharest. Beside 23 tram lines, there are two converted light rail lines called metrou uşor ("light metro"), numbered 41 and 32, which serve the western and south-western parts of Bucharest. Both use upgraded trams running on separate designated corridors for faster travel times. 67% of the city's tram