To create the "bubble" on the end...
First draw a path and set the end cap to be rounded:
Then use the Width Tool to make one end of the stroke wider, then still using the Width Tool, set a counter-width below the end point to constrict the path before the end. You should see the "bubble" form..
To add an "inline" stroke, use the Appearance Panel to add an additional stroke....
Click-drag the existing stroke to the New
icon at the bottom of the Appearance panel to duplicate it (this makes certain the width points duplicate as well). Then change the color and weight of the new stroke...
CS6 NOTE: You can merely add a new stroke in the Appearance Panel for CS6, the width adjustments will apply to new Appearance Panel strokes in CS6. In CC2017, the width adjustments DO NOT apply to new Appearance panel strokes [possible bug] - so you MUST duplicate the existing stroke and alter the duplicate rather than merely adding a new stroke in CC2017.
If you want to refine further, you can choose Object > Expand Appearance
an you'll have 2 shapes which you can then alter independently of one another.