well...
I suppose Karen is probably never coming back here, and may have found her answer on some other board, but just in case...
We do have a Stations, Terminals, and Yards forum under the Railroad History grouping, so this question should go there.
And Union stations, by their very name, were meant to consolidate the various railroads passenger operations of a given area into one main terminal (at least that was idea) - sharing a station for various benefits (only one 'main' station required (so less duplication of facilities and use of land area) - shared maintainence - convienent for travellers who need to transfer - oftentimes a civil landmark/showplace - central booking and ticketing... and so on).
Now, since 100+ railroad passenger operations have been replaced by, well, more or less Amtrak and commuter operations, Union Stations really don't deserve the name.
Besides the name having 'union' in it (not a good benchmark), I think one definition of a union station would be 'planned to support joint operations of two or more separate 'Long Distance' passenger services' (Long Distance to mean non-commuter in this instance, and services meaning different railroad companies from the pre-1970 mindset).
Nowadays, when you have passenger and commuter at a station, throw in Bus or LRT, and a day-rate parking garage and call it an intermodal center
Edit: I will throw in 2 links
One from the Straight Dope pretty much confirming my definition of a Union Station -
http://www.straightdope.com/classics/a800729.html
And one from Wiki, which is an (incomplete) listing of US Union Stations...
http://en.wikipedia.org/wiki/List_of_Union_Stations (alas, this list is based upon the word Union in the name, which I still think is a crappy benchmark)
