Learn Portuguese (Portugal): Subitamente, o maquinista apita e as portas fecham-se. - Suddenly, the train driver whistles, and the doors close.

This is the Historical Present (Presente Histórico). By switching from past tenses to the present tense right at the climax of an event, you make the story feel much more vivid and immediate, as if the listener is experiencing it in real-time.

Subitamente usually triggers the simple past because it introduces a sudden, finished action. Why doesn't it do that here?

Normally, a sudden interruption does require the Pretérito Perfeito. However, when using the Historical Present for dramatic effect, you intentionally override that rule, applying the present tense to those sudden actions to pull the listener directly into the suspense.

Could I say de repente instead of subitamente?

Absolutely. De repente is actually much more common in everyday spoken Portuguese. Subitamente is slightly more formal and literary, which pairs perfectly with the theatrical storytelling style of the Historical Present.

Why does the sentence use the reflexive fecham-se instead of just saying as portas fecham?

In English, you can simply say that the doors close, but Portuguese frequently uses a reflexive pronoun to express an action happening to an object when no specific person is doing it. This acts as a passive voice (passiva pronominal) to indicate that the doors close themselves.