Bobcat Tram Interurban Bus Service which transports students from Austin and San Antonio to the Texas State campus in San Marcos.
The BT buses can carry up to three bikes at a time. Though you will need to pay a second fare in San Marcos and change buses to get all the way to Austin, the total cost particularly if you are taking a bike with you is much cheaper than Greyhound or Amtrak (which won't allow a bike in any case).
The buses depart San Antonio from near the Randolph Park and Ride and make several stops along the way. In Austin you have a choice of drop offs. Of course since the service exists for students it only operates Monday to Friday and while the university is in session.
To read Daniel's complete article and to learn necessary information and insight into the service go to the Flickr group: Flickr-Club-SA. His article is entitled: A New Way to Austin.