Transportation Technology Center??

No secret.  It's the DOT/Association of American Railroads test track, and has been there since the 1970's.

Haven't heard much about it lately, I'm thinking their test budget was cut a while back?

Take a look at the Google satellite imagery: the track plan is a miniature bowl of spaghetti. In one area a number of rail cars are scattered around, maybe left over from crash tests? Could be a different idea for a model railroad theme.

DOT test track area

If you want to see a "secret facility", the Pueblo army base to the south has an extensive array of nasty storage bunkers.

A lot of the research and development work done at TTC isn't obvious.  Wayside defect detection equipment like WILD, acoustic bearing sensors and the like are pretty much constantly under development or refinement at TTC.

North American Rail Shippers had a meeting in Colorado Springs back around 2000 and attendees were bused down to TTC for an extensive tour.  It was easily one of the most fascinating visits I've ever made to a rail industry facility.  My only disappointment of the day was not having an opportunity to witness a crash test.

Curt

Here is some scoop about the different test tracks:

http://www.aar.com/tracks.php#WRM