The Chongqing Rail Transit (branded as CRT; also known as Chongqing Metro) is the rapid transit system in the city of Chongqing, China. In operation since 2005, it serves the transportation needs of the city's main business and entertainment downtown areas and inner suburbs. , CRT consisted of ten lines, with a total track length of . Lines 1, 4, 5, 6, 9, 10, the Loop line and Jiangtiao line are conventional heavy-rail metro lines, Lines 2 and 3 are high-capacity monorails. To keep up with urban growth, construction is under way on Line 18 and several other lines, in addition to extensions to Lines 5, 6 and 10. The Chongqing Rail Transit is a unique transit system in China because of the geography of Chongqing being a densely-populated but mountainous city, with multiple river valleys. Two lines use heavy-monorail technology, leveraging the ability to negotiate steep grades and tight curves and rapid transit capacity. They are capable of transporting 32,000 passengers per hour per direction however the busiest section of Line 3 reached a peak passenger volume of 37,700 pphpd in 2019. At , the system's two monorail lines form the longest monorail system in the world, with the Line 3 being the world's longest single monorail line even if the Konggang branch is excluded. The length and the capacity of its monorail network both also make it the world's busiest monorail system, with a total of 94 million and 250 million rides in 2015 on Line 2 and Line 3, respectively. The latter ridership statistic for Line 3 also makes it the world's busiest single monorail line. The extreme difference in elevation between the river valleys and the hilly plateaus of Chongqing pose a unique challenge in designing alignments for conventional rail transit lines. The network currently has the world's highest metro-only bridge, the Caijia Rail Transit Bridge for Line 6, spanning the Jialing River valley, with the bridge deck being approximately above the water. Hongtudi station is the deepest subway station in China and the second-deepest station in the world, after the Kyiv Metro's Arsenalna, with Line 10's platforms being more than below the surface. Liyuchi station, also on Line 10, is the second-deepest station in China, being below the surface. The Chongqing Rail Transit is also in the process of constructing a number of extremely-long metro-only suspension bridges. The long Egongyan Rail Transit Bridge carries the