I think if we're looking at it conceptually, it has to be something that is too complex to do with traditional heuristics reliably and also doesn't allow us to generate enough data for good DL results.

There's also liability to consider, for cases like airplanes and trains. Trains are dead simple to automate, but there needs to be someone there for long tail events, to make people feel safer, and as a fall guy in case of accidents. So in practice it's impossible to automate beyond subways where you control the entire environment despite the tech being fully capable of it. Same goes for airliners, they practically fly themselves but you need two people there anyway just in case.