Today, San Francisco is one of only five cities in the United States with an operational ETB fleet, but they play a major role in the Muni system, in part because of the city's many steep hills. Although their overhead wires are sometimes considered unsightly, ETBs are able to climb grades much steeper than conventional, non-cable streetcars and are quieter (particularly when climbing hills) and cleaner than diesel- or hybrid buses. The steepest grade on the Muni trolleybus system, 22.8% in the block of Noe Street between Cesar Chavez Street and 26th Street on route 24-Divisadero, is the steepest grade on any existing trolleybus line in the world, and several other sections of Muni ETB routes are among the world's steepest. Muni has operated trolleybuses since 1941 and the mode has been present in San Francisco since 1935—initially a line built and operated by the Market Street Railway and later taken over by Muni. Conversion of some existing diesel bus lines has been proposed. In 1992, Muni tested its first 60-ft articulated trolleybus, the New Flyer E60, which was the first in the trolleybus fleet to have a wheelchair lift. The E60s were used on high-ridership trolleybus routes and started service in 1993. Muni's active ETB fleet consists of articulated coaches from New Flyer (XT60), as well as standard 40 ft coaches from New Flyer (XT40). Historically, Muni ran ETBs from Brill, the St. Louis Car Company, Twin Coach, Marmon-Herrington, Flyer (E800 and E60) and Electric Transit, Inc. (ETI) (Skoda/AAI 14TrSF and 15TrSF). Battery-electric test buses In 2018, the SFMTA Board voted to purchase all-electric buses exclusively beginning in 2025, with the last non-electric buses retired by 2035. Muni previously had not bought battery-electric buses (BEBs) because they were not proven on steep hills and on high-ridership routes. In November 2019, Muni executed contracts with New Flyer (for $4.5 million), BYD Auto ($3.5 million), and Proterra ($5.3 million) to procure three BEBs from each vendor as a pilot program to evaluate their performance and test future bus features. Each contract has an option for up to three more BEBs. A fourth contract with Nova Bus ($4.8 million), the only major bus manufacturer excluded from the first round of contracts, was issued in April 2021. The buses in the pilot program will be charged at Woods using newly-installed chargers. Under the pilot program plan, the buses will operate on the 9