I'm sorry, there's no metro near the park. There may be bus service, but I'm sure that the schedules are all kinds of screwy.
I will be happy to pick you up at the Springfield Metro and give you a ride to the picnic. Unfortunately, I'll be heading south around 3:30 or so, so you'll have to get a ride back, which should be no problem at all.
FReepmail me so we can make the arrangements. I'd hate for you to miss a great party just because the Metro doesn't go to Mt. Vernon!