The next additions to the subway were surface commuter lines that linked to the north and east of the city. Line 13, a half loop that links the northern suburbs, first opened on the western half from Huilongguan to Xizhimen on September 28, 2002 and the entire line became operational on January 28, 2003. Batong line, built as an extension to Line 1 to Tongzhou District, was opened as a separate line on December 27, 2003. Work on these two lines had begun respectively in December 1999 and 2000. Ridership hit 607 million in 2004.