The tests are also such a small part in the recruitment process. They are simply the first and smallest hurdle. If you are not being successful in your applications for driving jobs perhaps you are not using the correct language. Many TOCs filter online applications before a human reads them and if there are not enough keywords (not specific rail terms but language such as safety, customer care, rules etc) then the computer says no straight away.
Unsurprisingly, you are spot on. until recently, I would have said that taking the tests privately would have given a candidate a significant leg up. I've had a trainee driver with me who had done them himself, as had others at that depot. However, with the use of automated recruitment, or the increasing involvement of HR types in the process (certainly the case at some toc's I'm familiar with), I think the advantage may be less than before.
Even if the TOC puts a candidate through the tests and they pass, they are most definitely not guaranteed a job. Plenty fail employer interview, and if there's two successful candidates for one job, they'll offer it to the one who scored highest throughout the process.
Every stage of the process is scored against certain competencies that they are looking for. Think communication, teamwork, flexible attitude, safety conscious, methodical, reliable, able to self manage, etc. If you're not getting an interview, review what you are putting on your application. There's been lists of the key competencies put on here before, so make sure that you are demonstrating all of them. Be honest with yourself. If you've not got the experience, work out how you could gain it, and re - apply when you have. Correct English and grammar is also important; one of the most important tasks a driver has to perform is report writing.