There is no tool at present for doing this directly, but what you can do is go to one of the individual vehicles that you would like to be part of your new (but very similar line), modify its schedule to that of the new line, and then select the "promote to line" button to create your new line. You can then reassign such of the other vehicles in the original line as you wish to the new line. If you did not want to modify the schedule at all, you might simply make a trivial alternation before promoting the altered schedule to a line (e.g., adding a waypoint) and then revert that alteration after it has been promoted to a line.