At the end of Monday's season finale she's shown getting on a train to Farhampton, holding a yellow umbrella and wearing the same boots shown earlier in season 8.
Which would line up with Ted meeting her at the train station outside of Barney and Robin's wedding.
The actress shown at the end of the finale is named Cristin Milioti and she's been on other TV shows like 30 Rock and The Good Wife. She was also nominated for a Tony in 2012 for her performance in Once.
I know, it's crazy, right?!
UPDATE: CBS has confirmed with BuzzFeed that Cristin Milioti is, in fact, the mother.