Metro Rail help please

Ok, this feels stupid, but how the heck do you PAY for your Metro Rail ride?

We are here for a few months for medical treatment, and my husband rides free, and I'm sixty cents. We got Q cards and I loaded money on mine, and we can get on the bus by tapping our cards on entry, but where do you pay on the train? Can't figure it out. We rode free today, but we didn't mean to! Thanks for any help!