I've had the opposite experience while job hunting here in the San Francisco Bay Area. Very few ExpressJS and mostly Rails for back-end requirements. The comparison to Sinatra is valid and I was able to spin up a quick/easy POC using in recent years. Your comparison makes sense to me and I share you preference for ExpressJS. When considering the old adage "choose the right tool for the job" I have to say that Rails is a better option when you want to do old-school CRUD operations using a SQL DB. There is no denying that activerecord fills a need that is pretty straight forward with very little pain or code (from our engineer) required. Apparently, there are similar solutions for the nodeJS world, but I have no experience with them and can't recall seeing job reqs that list them.
Finally, I want to point out that the "serverless" cloud solutions that I have seen, namely Google "Cloud Functions" and Amazon "Lambda" appear to have embraced nodeJS tech like Express, while rails lags behind. They do support Ruby, but that won't help you with your ROR app.
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Rachel,