I see all these parking lots all over the place by entrances to the interstates, but I’ve never seen instructions or costs. I know they are to encourage you to use mass transit, by not having to park at a train station but I can not find any information on them. I’ve looked at both the Metra and Amtrak sites. Do they shuttle you to the train? How often? Does anyone have more information on them? Thanks
Around here in NY, you just park. If there’s not enough spots, you’re SOL (but in upstate it’s only full around the holidays). You don’t have to pay anything for the actual parking, but then if your car gets broken into or hit or something they have no liability.
That being said, I have left my car there all day; albeit in the smaller towns, without a problem. I like them a lot. I use them to take the train to NYC because ain’t no way I’m driving/parking in NYC by myself. 
Are you looking to park at an Amtrak station, a Metra station or a CTA station? Here’s the CTA link:
I checked that link, and it does not link any of the Park and Rides I see along I-55 around Plainfield. If you can’t find the information how do they expect \you to know what time to get there. I think they are for the Amtrak to Chicago, but so far I haven’t found out for sure.
I heard they exist for the airports (or at least, for Midway). But no info on how they work.
The only two Amtrak stops in that direction in the Chicago area are Summit (near Midway) and Joliet. And Metra doesn’t really have service close to I-55 — there’s a line that runs parallel to it on the other side of the Ship & Sanitary Canal, but that’s about 3-4 miles away. There are some Pace (bus) Park and Ride locations in Bolingbrook; are those the ones you’re seeing?