Learn Farsi: او سوارِ قطار شد.

In Farsi, 'to get on' or 'to board' a vehicle is expressed with the compound verb سوار شدن (savâr shodan). سوار means 'rider' or 'mounted,' and شدن means 'to become.' So literally, the sentence translates to 'He became the rider of the train.'

Why is there an ezafe (the hidden 'e' sound) connecting سوار and قطار?

Because سوار functions as a noun here meaning 'rider' or 'passenger,' it needs the ezafe to connect it to the vehicle. You can think of سوارِ قطار (savâr-e qatâr) literally as 'the rider of the train.'

There is no Farsi word for 'on' in this sentence. Why not?

English uses prepositions like 'on' (the train) or 'in' (the car), but Farsi handles this entirely through the noun-phrase connection. Since you are saying 'he became the rider of the train,' no extra preposition like روی (on) or در (in) is needed.

How would I say the opposite, 'He got off the train'?

You would use the compound verb پیاده شدن (piyâde shodan), which literally means 'to become on foot.' The sentence would be او از قطار پیاده شد. Notice that unlike boarding, getting off requires the preposition از (from).