A Driver Managers job is "Not" above a Qualified train drivers job under any circumstances. A driver trainer is a different story.
Depending on what TOC you work for there are different ways the role is done. Some TOCS seperate the paperwork (line managers) and driver competency (development managers) roles completely where as some will have one person covering both roles.
In terms of the food chain in railway terms, driver is the safest job you can have. Any reforms or structural changes within TOCS leave all middle management vulnerable. Drivers will ALWAYS be required. You earning potential is much higher than your managers based on the fact you can do overtime too.
The traditional DM role, which combines the paperwork and the competency side of things, is above the driver grade in terms of seniority and basic pay.
Albeit the difference in basic pay is usually only a few £k and, as noted above, drivers who do overtime can earn substantially more than their managers.
Where the paperwork side is split out the DM role then becomes really just an admin role, with pay well below drivers’ basic pay in most cases.
Quite why some TOCs and FOCs see it as beneficial to split the role out isn’t immediately clear. I suspect it’s because it’s cheaper to employ more administrator “managers” on low salaries, and fewer competency investigators, who can command higher pay as qualified drivers.