Because the CTA isn't exactly the cleanest of places. Yes, I read it.
I fail to see how this is "trashy". Did you even read the article? It clearly states he chartered the train (which if you go to the CTA website starts at $2000 and offers additional services like music, refreshments, etc.) and she clearly loves it because she has a tattoo of a CTA map. As long as their guests are hosted properly, how is this different than renting any other non-traditional venue? A charter boat for the water lovers, a stadium for the sports lovers, a movie theater for the movie lovers, etc? It's not like they jumped the turnstiles and had everyone get on a public train on its normal route to get out of paying for a venue. It might not be your personal preference, but suddenly properly hosting guests (let's assume they did) in a personal and meaningful venue is trashy now?
As long as guests are properly hosted, I'm on board (pun intended) with an outside-the-box venue, especially if it's meaningful to the couple. I do understand where you're coming from @CharmedPam as far as not personally adoring a public transportation wedding. I once saw a woman going down on a man on the E train going from Manhattan to Queens (NYC subway). Yuck. Unless they were taking "first comes love, then comes marriage" literally.