Uml-modelling, inheritance or interface?

Hi all,

I have a scenario like that,

Engine Driver

•Captain : The Engine Drivers who reach “Enough Track Experience” and “Enough Year of Employee”. “Enough” terms can be changed in time. It must be flexible.
•Assistant Captain : The Engine Drivers who can not be captain yet.
•Certified Engine Drivers : The Engine Drivers who has a certification to operate High Speed Train. Either a Captain or an Assistant Captain is able to be a Certified Engine Drivers.
I want to show this relations using class diagram. But I couldn’t figure out how “track experience” and “year of employee” variables can be flexible. “Certified Engine Drivers” is another problem, because both captains and assistan captains can be a certified engine driver, so it sounds like a type conversion is needed? Thanks in advance…

I would identify use cases. You don’t say what the purpose of the system is. One possibility would be to have an EngineDriver class, and rank, certification as attributes?