$8 subway ride?

I’m afraid something went wrong here. When you use a credit or debit card to tap in, the ride should cost you €2,10 per ride with a maximum of €7,50 per day.
You write that you were alerted it cost you each 8 euros. How and where did you receive or saw this alert? When you look at your credit card account, were you actually charged €8 each?

See the website of the public transport company of Brussels for more information https://www.stib-mivb.be/article.html?l=en&_guid=a0faf6a3-4e98-3810-4da8-80cff5834093

From the Brussels transit site:

Why is there an amount of €7.50 blocked?
If you want to travel using the contactless payment system, an amount of €7.50 is temporarily blocked on your account. This amount is the maximum day fare. You could see this €7.50 booking appear in your payment record. This sum will be released once the correct amount has been debited from your account after 2 to 3 business days. Don’t worry: only the completed journeys are debited.
Don’t forget to always use the same card or device (smartphone or smartwatch), even if they are linked to the same account, so that your fare is correctly calculated. If you want to see your journey history, please create an account on our website.

So it appears your credit card blocked 7.50 EUR (~$8), which is the maximum daily transit fare. In a few days, it should revert to the actual fare of 2.10 EUR (~$2.25)

It’s confusing because the OP mentions both 8 euros and 8 dollars. If it’s in fact 8 dollars, this is indeed the equivalent of the maximum daily amount of €7,50. If you don’t take any additional metro rides today, the remaining balance of €5,40 (€7,50 -/- €2,10) will be debited to your account.

Thank you all very much. We were surprised when we saw the $8 charge- now we know. Much appreciated!