Perhaps it would be nice if I got a small verbal audio cue from whatever navigation app I was using a minute or so before reaching the rail crossing. We already get these notifications when it's time to make a turn and rail crossings aren't so frequent as to make such warnings annoying.

I could certainly imagine that if e.g. Waze warned about railroad crossings that it could prevent some small percentage of almost sleeping drivers/truckers getting hit.

It's a numbers game. 999/1000 will be watching just fine without a warning from their navigation app; that warning might bring awareness to 999.8/1000 or so. Or something like that. That could result in a pretty dramatic decrease of accidents.

This seems like a no-brainer to me - they should add a (visual) warning about this.

Is there a visual warning already? Like this for example (from Germany)?

https://upload.wikimedia.org/wikipedia/commons/thumb/4/43/Bl...

Sure, there are obviously visual warnings in real life, like that. Pretty sure that kind of thing is universal.

I'm talking about something that would pop on the Waze display, like 20 seconds ahead of reaching the crossing.

Why would this be an improvement?

It encourages taking drivers' eyes off the road. Can you offer an argument that its value is such that that's a net improvement?