There is a large risk of unnoticed sleep deprivation with sleep apnoea, even if you think you have slept for a reasonable length of time.
Yes, significant but unknown sleep deprivation could reduce ones concentration which is clearly a serious matter for train driving.
I was not aware of any connection between a high BMI and sleep apnoea though.
An unduly high BMI is undesireable lest it affects ones ability to easily fit into the cab, or indicates general health concerns.
AFAIK the BMI requirements for a new driver are stricter than for an existing employee.
30? years I applied to be a train driver and failed the medical due to overweight.