Someone who goes (from example) from a driver to a driver manager will already have a lot of relevant knowledge, but if they progress further they will find themselves managing people with very unfamiliar roles such as depot technicians. So, just like managers "off the street", they will need to have the ability and humility to engage with the workforce, listen to what they say, and be able to distinguish the sage words from the bull. Whether today's managers from either background have and use this ability is of course a very different question, but it probably does involve a lot of walking the job simply to understand what people do.