Learn Italian: Siamo arrivati in tempo per l'ultimo treno. - We arrived in time for the last train.

In this context, in tempo means you arrived 'in time' (before it was too late) to catch the train. If you used in orario, it would mean you arrived 'on time' or 'punctually' according to a schedule. Since you are talking about managing to catch a specific train, in tempo is the most natural choice.

Why is the auxiliary verb siamo instead of abbiamo?

The verb arrivare (to arrive) is a verb of motion that indicates a change of location. In Italian, these types of verbs require essere (to be) as their helper verb in the past tense, rather than avere (to have).

If an all-female group is saying this, does the sentence change?

Yes! Because the helping verb is a form of essere, the past participle arrivati has to agree with the gender and number of the subject. If an all-female group is speaking, they would say Siamo arrivate. The ending -i is used for a group of men or a mixed-gender group.

Why does ultimo come before treno instead of after it?

While most descriptive adjectives in Italian go after the noun, adjectives that indicate order or rank—like ultimo (last), primo (first), and prossimo (next)—almost always go before the noun.