The fact is that there are a limited number of controls/inputs. The biggest challenge would be retaining focus for the length of the journey, which I guess is a mental challenge of a variety but not the kind I was envisaging. From my perspective I don't see it as a 'mentally challenging' role. I can think of many medical, scientific and engineering roles I would consider challenging, but driving a train really isn't one of them, in much the same way as driving an HGV carries heavy responsibility but isn't mentally challenging.