The train has stopped — why?
'Fermato' — fer-MA-to. Stress on the second syllable.
Use this when the train stops unexpectedly between stations. You can ask a fellow passenger or wait for an announcement.
'Si è fermato' is the reflexive passato prossimo of 'fermarsi' (to stop). Reflexive verbs in passato prossimo always use 'essere'. The past participle 'fermato' agrees with the subject: singular masculine here. 'Perché?' at the end makes it a direct question.
Perché siamo fermi?
Why are we stopped?
First-person plural — includes yourself in the situation.
C'è un problema tecnico?
Is there a technical problem?
Guesses at the cause.
Quanto durerà la sosta?
How long will the stop last?
Asks for a time estimate.
Unexpected stops between metro stations are not rare in Italian cities. In Rome particularly, underground obstructions (archaeological remains, water mains) can cause delays. Announcements are often delayed or unclear. Patience is essential.