and automatic train supervision (ATS); in particular the ATP relied on transmission coils and wayside control units whereas the ATO relied on dwell operation control units. The transmission coils are controlled by the Control Centre to ensure safety of the line and were positioned on the guideway. Among such coils included the PD loop, safety frequency loop, stopping program loop, vehicle station link and station vehicle link; these loops were cross-arranged to produce electromagnetic induction with the interval between two cross points being 0.3 seconds to both monitor the train and control its speed. However this fixed-block ATC system used on the Muzha Line was plagued with problems in its early years of operation and was replaced with the new moving-block Cityflo 650 CBTC that was supplied by Bombardier Transportation of Canada for the Neihu Line. On the other hand, the heavy-capacity lines use the traditional fixed block system design, which were initially supplied by General Railway Signal of Rochester, New York, for the Tamsui, Xindian, Zhonghe, and Bannan lines; and later by Alstom for the Tucheng, Xinzhuang, Luzhou, Xinyi and Songshan lines. Key components of the system include impedance bond, 4-foot loops, marker coils, alignment antennae and two-aspect light signals for the wayside as well as automatic train supervision which utilises centralized traffic control. The Circular Line uses CBTC Radio signalling from Ansaldo STS. Public art In the initial network, important stations such as transfer stations, terminal stations, and stations with heavy passenger flow were chosen for the installation of public art. The principles behind the locations of public art were visual focus and non-interference with passenger circulation and construction schedules. The artworks included murals, children's mosaic collages, sculptures, hung forms, spatial art, interactive art, and window displays. The selection methods included open competitions, invitational competitions, direct assignments, and cooperation with children. Stations with public art displays include but are not necessarily limited to: , , , , , , , , , , , , , and . Stations with art galleries include , , , and . The promotion for artwork continues today – the Department of Rapid Transit held a bid on providing public large scale artwork for the interiors of . The bid is placed at over million. Other facilities In addition to the rapid transit system itself, Taipei Metro operates several public facilities such as underground shopping malls, parks, and public squares in and