The Yoker line is part of the Argyle and North Clyde lines. It is not an extension to a line but one of two routes between Dalmuir and Partick stations (the other route between them being the Singer line). The stations that lie on the Yoker line are:
As the shortest route between Dalmuir and Partick, the Yoker line is also the line that the express services to Helensburgh Central pass through (although these do not call at the stations).
References
Yoker line Wikipedia(Text) CC BY-SA