you can't unlink a card because they are kept on the account for verification, but what you can do is replace the payment method.

https://railway.app/account/billing

click the Replace button.

How do I do that if I don't have an access to the railway account where my credit card is linked?

Can you elaborate on why you no longer have access?

In the case that I would like to just remove my billing method without replacing it with another, what can I do? I do not wish to delete my account.

Can you elaborate on why you wish to keep a Railway account that is not on a plan? Without being on a plan you are not charged for anything.

But if I'm not going to be charged anything, why wouldn't I have the option to delete my payment method?

So, if in the near future I simply wanted to come back, I could do so with complete confidence that I could enter, pay for a plan and be able to unlink my card.

I find it a bit unfavorable that I can't delete my payment method. It seems to me that it's more of an option than an obligation. In any case, if I wanted to delete my account, the payment method would also be deleted, right?

Yes if you delete your account the data attached to the account will be deleted, though it may still exist in stripe's data.