The reason the PE Subway was filled-in, in the middle of its length, was because the City feared it did not contain re-enforcing steel in its original construction, to support the weight of the Bonaventure Hotel and other new Bunker Hill buildings. They were WRONG!!
I've seen the photos of the demolition. From the Bunker Hill side, they had to dig a long ways down, find the top of the tunnel shell, and then bash the hell out of it. It was quite a job, and the re-enforcing steel all showed up in the photos as they were punching thru the concrete.
Duh !!
By the way, did you know that "horsecars" operated thru the Subway ?? (only for a few minutes).
Check out the photos here:
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framework.latimes.com]
KRK