The Anchor exchange construction project had a cover story: an underground for Birmingham that, alas, had to be cancelled later because of changes in the market!
The main one in London was re-purposed from an abandoned underground station, itself used as a communication centre in the 2nd world war.
So a civil defence use for such tunnels sounds very plausible. No budget line, no need for any questions &c.
They are only done a few times a year, but at least it is offered. We also have lots of underground preprohibition brewery tunnel tours as well. There's a lot of old stuff in the city. However, it's only been the last 2 years or so that downtown has slowly been revitalized with life instead of abandoned buildings.
I was also amused to notice that the owner of the semi-finished subway system is 'SORTA'...Southwest Ohio Regional Transit Authority.
Buffalo built (and finished) a similar subway system in the '80s. At best, it didn't help.
Interestingly enough, it was actually used as a freight railroad until 1996. It is easy to walk right down into, with two different public access points that have no impediment to traffic. There is a lot of cool graffiti down there, especially in the area underneath the broad street bridge. I'd recommend anyone in the area to grab a flashlight and explore at some point.
http://videos.nymag.com/video/Mad-Men-Takes-on-the-2nd-Avenu... Men' Takes on the 2nd Avenue Subway
For comparison, Shanghai started from zero and built the largest subway in the world in 20 years.
SHANGHAI: Two men were killed and six others were injured here yesterday in two separate incidents at subway construction sites. 
5 dead and 18 injured in Shanghai Metro Line 12 construction site accident 
Not Shanghai, but...
On Nov 15, a subway tunnel under construction collapsed in Hangzhou, Zhejiang province, killing 17 people. 
After 20 years the Shanghai subway is now bigger than NYC yet they are scheduled to add a couple hundred additional miles.
The time and costs though - it makes you wonder how we used to be able to build things and now we can't so easily.
I can't find much of anything on the net but my Dad remembers it being built. In the seventies there was an entrepreneur who started running tours of the stations but he did so without city permission and they forced him to stop.
Also of interest, the current (and long beleaguered) Cincinnati public transit project, the streetcars: https://en.wikipedia.org/wiki/Cincinnati_Streetcar
Hopefully we'll actually get them this time.