I've never taken a bike on Metra, but I used to be a daily rider until late 2019. At that time, they allowed a couple bikes per car during non-rush periods. They might have loosened that restriction during the pandemic, as they are running well below capacity (my nonscientific observation while waiting on my bike for trains to pass at crossings). Folders can go at any time, though you can't obstruct the aisle.

You will need a way to secure full-sized bikes, typically using bungie cords to hold them to the side facing seats present in one half of each car.