Breakdown of В вагоне было тесно и душно, поэтому я расстегнула куртку, как только села.
Questions & Answers about В вагоне было тесно и душно, поэтому я расстегнула куртку, как только села.
Because it means in the carriage / in the train car.
After в meaning in, Russian normally uses the prepositional case for location:
- вагон = carriage, train car
- в вагоне = in the carriage
Compare:
- в вагон = into the carriage (motion, accusative)
- в вагоне = in the carriage (location, prepositional)
So here the speaker is describing the conditions inside the carriage, not movement into it.
Вагон usually means a railway carriage / train car. In this sentence, в вагоне most naturally means in the train car or in the carriage.
Depending on context, вагон can sometimes refer to other kinds of cars/wagons, but for a learner, train carriage is the safest interpretation here.