[UPDATE: To clear up some confusion in the comments, whatever net costs exist are borne by Sound Transit. As spokesman Bruce Gray explained:
The teams have never paid extra for this service. It’s part of our job to serve major events. Since the first Seahawks train, we’ve had great response to these services and have found it to be a great way to introduce Sound Transit to some who would never otherwise use transit.
Consider this a marketing or PR cost if you like.]
Sound Transit is expanding their special sporting event service on Sounder to cover all weekend day games of both the Mariners and the Sounders this year.
As I’ve observed before, sporting event service is a nice combo for rail transit: expensive parking, high congestion that traps most buses, focused arrival and departure times, and an opportunity to serve a constituency that may not have the occasion to use your service otherwise.