This tweet pretty much says it all.
It's $24 to park at Union Station for 12-24 hours, and there's no validation for Amtrak. If you park for a partial day, there isn't much relief, as 2-12 hours will set you back $20.
The official web page with current rates is here.
If you take a weekend trip to New York or Philadelphia (or wherever), leaving Friday afternoon, and returning Sunday evening, you're going to be shelling out a whopping $68 for parking (*)
Depending on where you live, taking Uber or a taxi will be both cheaper and easier.
Remember this post is here - it could save you a lot of money in the future.
(*) Please note that the website says, "Lost Ticket (flat rate) $24.00," so if you're gone for longer than a day, make sure not to lose your ticket, or you'll be costing the parking garage a lot of money.