A worn-out carbon insert would damage the overhead wire, stopping streetcar service. Apart from heritage streetcar lines, very few tram/streetcar systems worldwide continue to use trolley poles on vehicles used in normal service. Among the largest exceptions are the streetcar systems of Toronto, Ontario; Philadelphia (the "Subway-Surface" lines and Route 15); Riga, Latvia (however, new Škoda trams in Riga have pantographs); Kolkata (formerly Calcutta), India; and Alexandria, Egypt. Smaller systems still using trolley poles for regular service include Hong Kong Tramways, the Daugavpils, Latvia system, and Rio de Janeiro's Santa Teresa Tramway. The MBTA system of Boston still uses trolley poles with the PCC streetcars it uses to serve the Ashmont–Mattapan High Speed Line. Compatibility with pantographs Trams or light rail cars equipped with pantographs normally cannot operate on lines with overhead wiring designed for trolley-pole collection. For this reason, these systems and a few others worldwide retain use of trolley poles, even on new streetcars, in order to avoid the difficulty and expense of modifying long stretches of existing overhead wires to accept pantographs. However, the Toronto Transit Commission, with the impending replacement of its legacy CLRV and ALRV with new Flexity Outlook cars, is converting its overhead power supply to be compatible with both trolley poles and pantographs on an interim basis, as the CLRVs and ALRVs use only trolley poles while the Flexity fleet is equipped for both trolley poles and pantographs. Starting September 12, 2017, streetcars on route 509 Harbourfront started to use pantographs while those on route 510 Spadina continued to use trolley poles along the same section of track on Queens Quay West. (On May 14, 2018, 510 Spadina switched to pantographs.) Large portions of San Francisco′s surface network are also set up to handle both trolley pole and pantograph operation in order to allow for compatibility both with Muni’s current fleet of light rail vehicles (pantograph only), as well as Muni’s historic streetcar fleet (trolley pole only). Cultural references Upon their introduction, trolley poles and the new electrical technology they represented were fascinating to writers, with their lightning-like sparks and power. In January 1889, Boston introduced its first electric streetcars, which became so popular and noteworthy that poet Oliver Wendell Holmes composed a verse about the new trolley pole technology, and the sparking contact shoe at its apex: In 1947, composer Samuel Barber wrote the now-classic orchestral and vocal piece Knoxville: Summer of