I had a similar situation happen to me. I went to the interview and they said that there were 2 openings for the Feb class and the rest of the openings (15) would be for April. The HR person tolds us at the interview that if we are selected for the April class then we would receive an email stating we were selected as an alternate. Well, the next day I got an email stating I was an alternate, like you. A few weeks pass and I called about that April class because I hadn't heard anything in a while. That class got moved to June and that I should wait because the application stays active for 6 months. They posted another job advertisement in March for my location, so I naturally thought I was good to go since I already have an alternate selection.
Turns out I was right. I got my conditional offer on Friday, 4 April 2014, and I am starting the pre-screening process today.
This was for a conductor's position in San Antonio, TX