Is it still OK to wear trainers with a suit when commuting?

[GodAtum]

I see many men in suit wearing trainers on the train, I assume they must change before they get into the office? Or do they change when they sit down at their desks?

 

[SteveMc]

I see many men in suit wearing trainers on the train, I assume they must change before they get into the office? Or do they change when they sit down at their desks?

I have many reasons to believe they don't change. Most likely, they simply chose function before form.