Railway does not have a free plan. If your subscription happens to be waived, it may be waived the following month should your account remain sufficiently active

Once my $5 is gone, will it never be renewed again? At least you buy a plan?

As jake said, there is no free plan, there is only a trial plan, and the trial plan gets you $5 worth of credits only once, there is no expiry on those credits but once they are used up your services will be shut down and you will then need to upgrade to the hobby plan if you wish to continue using the platform/