Learn Portuguese (Portugal): Temos comboios rápidos.

In Portuguese, adjectives generally come after the noun they describe. Instead of the English order "fast trains", you say the equivalent of "trains fast" (comboios rápidos).

Why does rápido have an "s" added to the end here?

Adjectives must match the noun they describe in both gender (masculine or feminine) and number (singular or plural). Because comboios is a masculine plural noun, the base adjective rápido changes to its masculine plural form, rápidos.

Where is the word for "we" in this sentence?

The word for "we" is nós, but it is usually left out in Portuguese. The verb form temos already means "we have", so the pronoun is understood without needing to say it.

What is the purpose of the accent mark on rápidos?

The acute accent tells you to put the spoken stress on that specific syllable. When you say rápidos, the strongest part of the word is the first syllable.