Learn Portuguese (Portugal): O comboio parte às oito.

The infinitive (dictionary form) of the verb is partir (to leave or depart). Because the subject, o comboio (the train), is third-person singular (acting like 'he', 'she', or 'it'), we drop the -ir ending and add -e, making it parte.

Why do we use às instead of just a (at)?

In Portuguese, you do not just say 'at eight'; you literally say 'at the eight'. The word às is a contraction of the preposition a (at) and the feminine plural word for 'the' (as). The grave accent mark on top shows that these two words merged together.

Why is the feminine plural word as hidden inside às if o comboio is a masculine singular word?

The 'the' inside às does not refer to the train! It refers to the time. When telling time, Portuguese implies the word horas (hours), which is feminine and plural. So, às oito is actually short for às oito horas (at the eight hours).

I have sometimes seen the word trem used for 'train'. Why is it comboio here?

comboio is the standard word for train in Portugal. If you have seen trem, you were looking at Brazilian Portuguese.