Their journey usually involves walking to the station, walking to the escalator, walking from the escalator to the train, and walking from the train to work.

That short escalator ride isn't a public health disaster.

The lack of healthy food available to commuters is probably a bigger problem.

It's more subtle than that. The difference in attitude between "I walk when it's unavoidable" and "I walk whenever I can because we spent a billion years evolving to do it and living in a city is very bad for my overall health and wellbeing".

There was a woman who became obese following a transplant of gut flora.

Recall how much the idea of evolution bothers some people. Are you sure this isn't a similar type of situation?

We seem to have evidence that obesity isn't always a choice.