Traditionally drivers were 'superior' to guards, cleaners, passed fireman, porters and so on.
All grades used to work like that, and there used to be a strict hierarchy. Drivers would start as cleaners, and there used to be 'links' of drivers as well. You would be well into your 50s driving top link expresses.
Preserved railways still operate like this.
The idea was really to select the best of the best going through the different grades, in practice people were promoted on seniority IE how long they had been in a job, a lot of the time.
All this has tended to go by the wayside after privatisation, and drivers were recruited 'from the street' - which led to their nickname 'boil in the bag drivers' and they could be driving trains in little more than a year.
A lot of TOCs still prefer people to start off as guards because they can assess whether they are a team player, can handle pressure and responsibility, are reliable, and so on.
There are still grades in other jobs such as signallers, Pway and S&T. This all reflects the way people work up and learn on the job as they go, and gain enough experience to take on greater responsibility.
The railway has always, and continues to keep a record of everyone's discipline (IE breaking rules - say for example during an engineering possession or single line working - and being involved in incidents such as SPADs) and 'black marks' in terms of sickness, timekeeping, attitudes, exam results (exams are taken in many grades every few months/years or on application to a new job) and so on. Most people never, ever see their records. The records are sent to new managers on applications for promotion to a 'higher grade'.
If a guard has a poor record he is unlikely to be promoted to a driver. Your reputation and chances of promotion are made/broken on the job. There are many cases of people never being promoted because they are considered not up to the higher grade - this is assuming you can get on the railway in the first place. Often 3-400 people for each railway job these days.
Normally staff in high grade signalling posts, S&T, Pway, and the more selective TOCs like Eurostar and Virgin take many years to get there and you can be demoted as well in some grades.