What I've tried: (1) clicked on the "unsubscribe" link at the bottom of their emails (did this numerous times on several emails), (2) logged in and changed my account's email to a fake email from temp-mail.org, (3) deleted my account, and (4) emailed a Railway support engineer who told me last month that they've deleted my account data.
I understand that I could set up a Gmail filter to automatically delete all incoming Railway emails, but I'd like to avoid this approach. I might use Railway in the future and will likely forget about this filter.