U2 cars use Scheren (diamond) or single-arm (z-shaped) pantographs, while Calgary, Edmonton and San Diego vehicles use the latter style pantograph exclusively. In order to operate safely, the cars require to accelerate from a station, and to maintain speed. Variants Siemens-Duewag Type U3 is an upgraded version of the U2 with a slightly longer length and cosmetic changes. Introduced in 1977, they entered service in 1980 and will be exported to Monterrey in Mexico, after being retired from service by Germany's Frankfurt U-Bahn. Three examples will be preserved for museum service Siemens-Duewag Type U2A on Sacramento Regional Transit's light rail system is an upgraded version of the U2 that shares similar characteristics of the newer SD-100s and SD-160s, yet it still uses the mechanical equipment of the U2. Siemens also developed a U2 running on alternating current motors using inverters (nicknamed the U2-AC). 2 units were purchased by the Government of Alberta in 1988 and operated as demo units for Edmonton and Calgary. Calgary ended up adding the two units to their main fleet until 2015, when both were retired due to part issues. One of the units is still active as a track inspection vehicle, while the other was scrapped for spare parts. Two variants, designated U2h and U2e, were locally created during the early 2000s in Frankfurt to serve different platform heights on the network. Originally the trains were designed to serve platforms of height, with one large step at each door. As platforms were rebuilt to create a unified standard height of , modifications were required to the fleet: U2h retained a small step to serve platforms on lines U1 to U3, while U2e lost the step entirely to serve and later platforms on lines U4, U6 and U7. U2 cars acquired, by city Calgary (C-Train) — 85 cars (83 U2-DC, and 2 U2-AC acquired from Edmonton) Edmonton (Edmonton Light Rail Transit) — 37 cars, 2 U2-AC cars operated as demonstrators between 1988 and 1990, plus 3 parts cars acquired from Calgary. Retrofitted to have electronic LED destination signs on the exterior and interior among other changes, including introducing moving block communications-based train control systems installed by Thales which allow the trains to run in a fully automated mode Frankfurt (Public transport in Frankfurt am Main) — 104 cars; U2 cars completely retired from service as of 2016. San Diego (San Diego Trolley) — 71 cars; U2