Be Able to Start A New Line At or On an Existing Line or Object
Currently, new lines have to be started a short distance away from any existing lines or features. This causes a glitch in the professionalism of maps because there are "stub outs" that have no purpose and always raise questions by map recipients.
Bill May commented
Easier-- use the Distance tool, which can be used to place points anywhere, then convert to Route (right-click on the Distance line)
While you can not currently create a new layer exactly on top of an existing one, version 9.5 (coming soon) will have some improvements in this regard.
Moreover, assuming that you are using the Route tool to create lines (and if not, please consider using it instead of the Track tool as it is far more flexible) you can create the first waypoint in the route in the area near where you want the line to start, then click and drag that waypoint to the desired position (including on top of any existing layer.) You can use this technique to create the exact effect that you desire. Once you are done with the route, edit it to hide waypoint name and symbols - if so desired.
Check the help topics on routes for more ideas and tips.