The canal under Xinsheng joins another canal just north of where the old Guanghua Market was. From there you can follow it to the west and south, paralleling Civic Boulevard for a while, where much of it is a park called 瑠公圳公園 (Liugongjun [Liugongzhen] Park). It proceeds across Fuxing S. Rd. behind Sogo, then diagonally across Zhongxiao, making up that diagonal plaza you see there. Then it proceeds past Eslite, along Anhe Road, and then diverges diagonally from Anhe Road to the Guangfu/Xinyi Road intersection. From there the canal narrows and heads southward. South of Furniture Street, it joins another canal flowing from around the Taipei 101 area. Then it goes south all the way to Jingmei, connecting to more canals and the Jingmei River. The whole canal system was called 瑠公圳 (Liugongjun [Liugongzhen] - see the name of the park above), and it was an important means of transporting goods in the olden days.
There are some places where the canal can still be seen. Certainly you can see it under the overpass at the north end of Xinsheng. I wonder how the MRT construction projects deal with it.
One of these days I’m going to walk all the way along it to Jingmei - call it “urban river tracing”. It’ll be good exercise!