How much is enough ? There are reasons why there is such a high degree of competency management. It is keeping the railway a safe place and ensuring the people who work there (not just the Drivers) are up to a safe standard.
You are either working or not. If you are going to do a few shifts a month then you are still very much part time and employed by the company. Part time and reduced hours is something that is already happening. TOCs already have links where people only work 2/3 days a week and even restricted working hours; such as an 'old mans link' The question becomes how much can that be extended and at what point does it become unsafe.
Coming back on an ad hoc basis after leaving isn't plausible, sensible or safe. Consider the medical standards alone. If you were retired or casual for a few months and got sick or took medication or needed glasses then you no longer conform to any medical standard. As an employee I am required to inform my TOC any time I'm sick or need anything medically related. This is recorded and should I have something as simple as conjunctivitis I would need a medical before I was allowed to drive again. Same with hearing infections you need a medical before returning to work. If in the odd few months I was off work and my eyesight deteriorated, what would happen if I came back to drive and was under the eyesight standards ? My glasses would also need to meet the TOC and RSSB standards. As an employee, If I need to take any medication of some sort I have to declare it. Certain medications, again, need to be reviewed by occupational health and given the all clear before I am allowed to drive.
The cost of employing Drivers and maintaining their competency and standards is immense. For someone who chose to retire or left the company then I doubt very much the company would accept that cost.
What would be possible and IS happening is that you could employ part time Drivers or employ specifically for restricted hours. The cost would also be exorbitant but it would be possible. TOCs already do this with people who are slowly moving towards job shares etc. But they are still very much employees. Any suggestion that people can leave and return on any kind of casual basis is pushing the realms of fantasy.
TOCs are very sensitive to costs. They rely on overtime because it is cheaper. Even if there were part time and casual workers, they would still need overtime because it would be cheaper.
A thought just popped in to my ickle head. If you had a 'casual worker' who only worked 1/2 days a month. Isn't that much the same as someone working overtime ? The TOC is just as reliant on someone working a few extra hours a month on an ad hoc basis. Almost the very definition of overtime. :/