This is exactly what I was trying to do. I didn't realize you could manage the offsets and tapers in the offset alignment properties!
Consider this one solved!
Cheers to all,
He can just graphically slide the transitioning grips to change the Transition Segment length if he changes the length of the Parent alignment.
You can do this also with the help of the Subassembly Composer and some API calls, no Offset alignments required.
Your alignment drawing worked perfect. Could you explain how you actually went through the steps of creating the offset alignment and then tweaking the widening. Did you use some type of constraint or do it manually?
Start by creating a very basic centerline, just a straight line from your proposed start point to end point to get started. Before you create any offset alignments you likely need to edit the default settings for the Add Widening command. Otherwise your transition segments through curves don't work very well. The default settings can also cause the command to fail, because the segment and transition lengths won't fit within the overall length of your alignment. Here's what I'm using for the example drawing.
Use the Create Offset Alignments command from the Ribbon then select one of the new alignments and use the little cyan + icon to create the default widening. Once that's done you can select the alignment again and pick on the white dot in the middle of the transition. This should turn on two new cyan grip points which allow you to drag the start and end of the transition.
You cannot move them to the absolute ends of the centerline alignment (if you do then the transition disappears and you have to recreate it) but you can use the station-offset transparent command to get them close enough. The idea is to have that one transition cover the entire length of the centerline.
Once you have all of that done you can go ahead and start editing the centerline to add PI's and curves etc. Note - you can also try to do this step first, but I've sometimes had difficulty creating the widenings if the centerline already has complex geometry.