Not sure if they are still available, but a monthly pass at the Stamford garage is $70/month. Not sure how much daily parking is, but it’s not much more than $8, I think. Driving to Greenwich to take the train involves I-95 which would seem to defeat the purpose of taking the train. You could drive to Noroton Heights and park there for $4/day. And because you are catching the train before it reaches Stamford, you have a better shot at getting a seat on the train. Of course, the train doesn’t come as frequently at Noroton Heights. Good luck!