What on earth does university education have to do with intelligence? The drivers tests are for innate skills rather than things you pick up through university, that's why anyone can get the job off the street from any walk of life.
If anything I'd suggest being a decent guard requires more intelligence and 'life skills' than being a driver, as you end up having to work your way through more varied situations with the ever variable human factor. Companies nowadays want their drivers to effectively drive like robots in a set way with good reaction skills and a bit of common sense. That's not to dismiss the driver's job, far from it, but the two jobs require similar skillsets in some ways (calm under pressure, logical reasoning and all that) and in other ways totally different so good candidates for one may not be for the other. From my experience though I'd exclude more drivers from guards positions than I would guards from drivers.
As an example, I was recently working a train which couldn't get a brake release. Having conferred with the driver, checked for passcoms and MCBs and the usual, they called for a fitter. I happened to notice the main res air had dropped to slightly below the threshold for brake release (the train was full and standing and I reckon blowing the airbags up had just dipped it below) so I got the driver to switch on the compressor speed up and lo and behold, away we went. Either of us could have noticed that and sorted it and it doesn't require a brain surgeons qualification, just logical reasoning and working as a team we cracked it with minimal delay.
However on an other occasion I was working a train with a passenger having a nervous breakdown having just been told their parent had been found dead and then lost their phone signals. I put in place various processes to ensure their well-being and generally took pretty good care of them (despite her state she was kind enough to write a letter to my manager thanking me a little while later). I even, despite some folks shrieking about never touching other humans in case you have fingers chopped off and get thrown in prison, gave her a hug and held her for a while. Train drivers are not necessarily assessed on those kind of direct people skills, particularly in situations that are as distressing as any other emergency, and indeed in plenty of cases I know those people skills just aren't there.
I would also far rather have a driver who has lived a bit of life and gained experience as something like a police officer or a soldier, or even a train guard (

) than someone straight out of university.