Hi Bravlin, it's usually best to do such edits on your unbent shape and then do a new Flow with your updated object.

It is possible though to do a kind of "scale from center path" operation with Flow, to do that you would make a Sweep of a line profile:


Along a central rail curve:



That will be the base surface for the flow. Then make a narrower sweep surface using a shorter line along the same rail, that will be the target surface for the Flow. An example result is attached.

- Michael