That siding on the south bound side just south of Armitage

Is it used for anything?
There is a (sort of) matching structure on the other side, but no track.
What is the story on this bit of the L?

I think I can see from the underlying structure that it was part of the original structure before the portal. Is that correct?
Was it ever four track south of the portal (after the portal was built, obviously it was previously)?

Thank you.

I believe those tracks leading south from Armitage that fan to the outside were once used by North Shore trains; with CTA trains using the two inside tracks. Here's a great source for a more detailed historic explanation...

Thank you.

Information at that link answers all my questions but one.

Anyone know if that siding is used for anything, or what it was used for at some time after the rest of the track was cut back?

I don't believe it's used anymore. I'm somewhat surprised CTA didn't use the additional space during the Brown Line rehab to soften that curve. Even if it couldn't be softened enough to change speed restrictions, it would have an impact on wheel damage.

I used to live a block away from this siding. I don't recall ever seeing anything parked there. I believe the outer tracks were removed from service at the time the subway opened, but before the CNSM quit. I say this because I once read in First & Fastest of a CNSM southbound that was mistakenly routed onto the outer track. It proceeded all the way to Chicago Avenue but could not regain the inner main as the switch was frozen. Upon contacting the dispatcher, he was in shock that the train made it that far as I didn't realized he had lined it up for the outer main. The train had to backtrack all the way to Armitage to cross over to the inner main.

As for the curves, they aren't bad at all from a rider's perspective until you get to the S-curve over North Ave. Given that you have parking lot on both sides of the road, it would make a lot of sense to rebuild this as a diagonal rather than a formal S-curve. Trains crawl through here and the flanges squeal like crazy.

The timeline on the Chicago-L page linked by doepack above is:
1961 - All trains south of Armitage routed on tracks 2 and 3 (inner tracks)
Jan 21, 1963 - North Shore Line abandons service
April 12, 1964 - Tower closed
April 22, 1968 - Interlocking removed - hand throw switches align 2 and 3 with the subway and 1 and 4 with 2 and 3 south of Armitage
October 18, 1976 - Tracks 1 and 4 removed from service (were used during maintenance on tracks 2 and 3)
Several track changes occurred with the interlocking restored May 7, 2001 - initially controlled from a relay house at Webster Ave

Your First and Fastest story could have occurred between 1961 and 1963