When you touch on an existing feature in collector you can do a variety of operations on it: edit, zoom to, delete, etc.
One of those operations is 'Directions to Here'.
As far as my testing can tell this is only available for point and polygon features. It is not available for line features.
Yes if you view the map in ArcGIS online and click on a line feature you can pick 'Get Directions' from the popup.
Can someone at ESRI confirm this that in collector the 'Direction to Here' is not available for line features.