Hey there fyzix! Angelo here, Support Engineer from Railway. I was here procrastinating from work when I saw this comment.

The downtime you see is our proxy taking time to cutover- however, we have https://docs.railway.app/deploy/healthchecks that will only cutover once we have a 200 from your API. This way we can keep your old deploy live and serving requests if and only if it's live.

Theres more we can do to make it magical, but this should help in the meantime.