If what you say is true- if you needed the money, why would anyone want to work at a lower grade if they were more than capable of working at a higher one?
Never make the mistake that you are more than capable at any level. To cock assure on the opps floor and your colleagues will just leave you to swing by your own rope, more humble and ability to learn from your peers will win you respect as well as your ability to deal with complex failures and engineering work.
In the nearly 20 yrs I've been a grade 9, we have not had a single recruit from signalling school that has arrived at our place prepared for the share intensity of the level of work, not to mention there are huge gaps in their knowledge base at a basic level that makes teaching a new recruit very stressful compared to a promotional move.
But also we have had some signallers who have won promotion who were punching well above their weight that needed a lot of guidance to make you feel confident to work along side them.
At the moment we seem to have a one size fits all approach to recruiting signallers from grade 1 to grade 9, compared to old test that put the recruit into capability bands level 1 = grade 1 (crossing keeper), level 2 = signaller grades 2,3 & 4 and level 3 = signaller grades 5, 6, 7, 8 & 9. The grade or rate of pay you get as said elsewhere is based on the grade of the signalbox/centre you work at not how well you did passing the acceptance test or signalling school tests. If you feel your better than you have to apply for promotion (after your year of probation) along with everyone else, that is the current system.