Can you keep my seat? I'm going to the toilet.
te-NER-mi — stress on second syllable. 'Bagno' is BA-gno.
When sitting next to a friendly fellow passenger and needing to leave temporarily.
'Puoi tenermi' uses 'potere' in informal 'tu' form + 'tenermi' (keep for me). This informal question is appropriate after you've established a friendly rapport with the person next to you. 'Vado al bagno' (I'm going to the toilet) explains why you need the favour.
Può tenermi il posto? (formal)
Can you keep my seat? (formal)
The formal 'lei' version with 'può' instead of 'puoi'
Ci sei tu — torno subito.
You're here — I'll be right back.
Very informal — assuming the person will stay
Non si sieda nessuno qui — torno tra due minuti.
Don't let anyone sit here — I'll be back in two minutes.
Said to a stranger in an open seating context
Italians are generally helpful with small favours like keeping a seat. The informal 'tu' form is used with someone your age or younger, or after some friendly conversation. Use 'lei' (formal) with older passengers. It is considered impolite to leave your luggage on a seat for long periods on a crowded train.