Well I went there this past Friday afternoon so thought that perhaps someone else might be interested in knowing more about it.  That is especially since I had read about it on here somewhere.  It is located at the end of the Blue Line, #4.  This would be Anpyeong Station.  I would suggest getting there no later than 1600, since it closes at 1700 and it is not actually at the station; after asking to tour it (Which I did by pointing to the picture and caption of it on the city map.) after which they will put you on a train going towards their yard, and get off about two minutes later where you will be signed in and given a badge.  Also you should have some form of identification with you because it will need to be retained while you are touring the site.  It was not as comprehensive as I expected, covering mostly just that line.  There are refrences to other modern and similar systems, but that was about it.  The oldest one represented was perhaps one in Germany dating to 1901.  The whole thing is mostly virtual or photographic I might mention.  There is also some type of theme park which I did not tour due to the damp weather, length of time and being by myself.  This might have more hands-on or life-size exhibits.