The participants in the train study were essentially given a "job" to talk to strangers, which completely changes the mental framing.

People generally try to follow through on a job they promised to do. They'll try to make the best of it so they feel like they made a good decision accepting the job.

If the conversations go badly, they can easily rationalize it away with "I didn't want to talk to them anyway, but I didn't have a choice".

Many shy and socially anxious people do basically fine in public-facing jobs because of this phenomenon.