The thing is, if you know enough trips to unmask them, then you can find out about all their trips in that dataset.

As an employer, maybe I can find out an employee wasn't home sick when they said, but took the bus to a station that only serves a competitor's business. Etc.

This is a really good point. I hadn’t fully considered the different ways you could know just part of someone’s routine.

Its almost any two trips. Its the exception that two people take the same trip together. I can think of a handful of people I could eventually find 2 trips for who wouldn't want me to have their entire travelling history.

One could trivially establish multiple trips that anyone with even slightly public social media has taken.

Ok this is an interesting point. I can’t tell if this is a challenging problem or not. It’s probably worth trying an experiment.