Learn Portuguese (Portugal): Ela está chateada porque perdeu o comboio. - She is upset because she missed the train.

You're right that the verb perder usually means 'to lose'. However, in Portuguese, it is also the standard verb to use when you miss transportation, like a train (comboio), a bus (autocarro), or a flight (voo).

Why do we use está instead of é for 'she is'?

We use está (from the verb estar) because being upset is a temporary feeling or mood. If you used é (from the verb ser), it would imply a permanent trait, meaning she is generally an annoying or grumpy person by nature!

Why does chateada end with an 'a'?

Adjectives in Portuguese must match the gender of the person they describe. Since the subject is feminine (Ela), the adjective takes the feminine form chateada. If we were talking about a man, it would be Ele está chateado.

I've sometimes seen the word 'trem' for train. Is comboio different?

comboio is the word used for 'train' in Portugal. You will often see 'trem' in dictionaries or resources that teach Brazilian Portuguese, but in European Portuguese you should always use comboio.