Outline path not working

[PeterD-NJ]

Urgent request that's stopping publication: I am the creator of a railway track map book. Until today, I was able to use Object->Path->Outline Stroke to take a line (i.e. an open path), and turn it into a closed path that I can apply a stroke to. This is required to give a thin 0.1pt stroke to a faint yellow line so it will be visible on the printed page.

When I use the same process I've used for years now, the path in the middle of the stroke remains of the same appearance, but with infinitely more handles, plus a few of what look like small "bullseye" style symbols appear as well.

If I draw a line with the pen tool and stroke that path it works, but if I use any existing line, also created with the pen tool, it fails. 100% repeatable and it's preventing me from going to press. I'm willing to hire someone, if necessary, to figure out what's wrong here because without this feature I cannot publish.

This is how it looks after applying the Outline Stroke feature now. It looked the same "before," except far fewer handles. Yet if I create a line immediately beside the path, in the same layer, it behaves correctly.

And from the exact same file, with absolutely nothing changed or different earlier today:

This is precisely what I need to do with yellow lines, and it's worked the same way for many years. Help! Please!!

[Ton Frederiks]

Did you check the layers panel and appearance panel for differences with your normal strokes (maybe it is a brush) ?

As a workaround try to copy them in a new document, outline them , apply the thin stroke and paste them back.

Or add a new thicker black stroke below them in the Appearance panel.

[Monika Gause]

please share a demo file before and after.

Change the file extension to .pdf, then you can upload it to this forum.

[PeterD-NJ]

Here is a demo PDF containing a very small sample of the document. There are 3 identical  (copy/pasted) sections; The first is the original, the second, after "Outline Stroke" was attempted; the third, after "Merge" and "Outline Stroke" was attempted, and freehand drawn with Outline Stroke attempted. None of the attempts were successful.

[packaging specialist]

Object->Extend Appearance + Object->Path->Outline Stroke 

[PeterD-NJ]

It's a partial success but still does not yield the correct output, even using Control+8 (merge):

That line/break should not be present; this should be one shape. Here's how it looks in the first version of the maps drawn many years ago:

And while this segment is correct (and a couple of others) in the old map, the geography is so badly off I cannot use that file.

[Monika Gause]

If you want one shape, then making a compound path with Ctrl+8 is not correct. You want the Pathfinder Unite. From the PF panel, not the effects.

[PeterD-NJ]

I've never heard of that, but for many years Ctrl+8 has given me the desired effect. Is there a benefit of using the PF version instead?

[Monika Gause]

To me it looks like you are confusing things here. It sounds like you want to do this: https://youtu.be/fFvgEANuwN0

You need the Layers panel (Ebenen in the video)

And the Appearance panel (Aussehen in the video)

And the Effect Outline stroke (Konturlinie in the video)

[PeterD-NJ]

That is definitely not the right tool for what I want to accomplish. As I test, I selected the lines in the top-most image in that test file, and applied Pathfinder's "Unite" command. This was the result:

[Monika Gause]

You need to first outline the strokes.

[Ton Frederiks]

I tried : Object > Expand Appearance

Object > Path > Outline Stroke

Pathfinder panel > Unite

Add a Stroke to the result