O trem está parado na estação.

O means the, while um means a/an.

So:

• o trem = the train
• um trem = a train

Portuguese uses definite articles very often, just like English, but sometimes even more naturally than English does. In this sentence, o trem refers to a specific train, not just any train.

Because trem is a masculine noun in Portuguese.

That means it takes:

• o in the singular
• os in the plural

Examples:

• o trem = the train
• os trens = the trains

Grammatical gender in Portuguese does not always match biological sex or make logical sense to English speakers. You simply learn the noun together with its article.

Portuguese has two main verbs for to be: ser and estar.

Here, está is used because the sentence describes a state or condition:

• the train is stopped
• the train is at the station

These are seen as situations, not permanent identity.

Compare:

• O trem está parado. = The train is stopped.
• A estação é grande. = The station is big.
  (a more permanent characteristic)

So estar is the natural choice here.

Parado means stopped, motionless, or stationary.

In this sentence, it describes the train’s condition: it is not moving.

It comes from the verb parar = to stop, but here parado is functioning like an adjective.

Compare:

• O trem para na estação. = The train stops at the station.
  (para = verb)
• O trem está parado na estação. = The train is stopped at the station.
  (parado = adjective/state)

Because parado must agree with trem, and trem is masculine singular.

Agreement in Portuguese affects adjectives and many past participles used as adjectives.

So:

• o trem está parado = the train is stopped
• a locomotiva está parada = the locomotive is stopped
• os trens estão parados = the trains are stopped
• as locomotivas estão paradas = the locomotives are stopped

Na is a contraction of:

• em = in / at
• a = the

So:

• em + a = na

That gives:

• na estação = in the station / at the station

Other common contractions:

• em + o = no
• em + os = nos
• em + as = nas

Examples:

• no trem = on/in the train
• na estação = at the station

Estação by itself often means station, and context tells you what kind.

If you are already talking about a train, na estação is usually enough.

But you can say estação de trem if you want to be more explicit:

• O trem está parado na estação de trem.

That is grammatical, but often less natural if the context already makes it obvious.

In many situations, Brazilians may also say:

• estação ferroviária = railway station
• estação = station, when the meaning is clear

Yes, trem means train in standard Brazilian Portuguese.

However, there is an interesting regional note: in some parts of Brazil, especially Minas Gerais, trem can also be a very general informal word meaning something like thing/stuff.

For example, in casual regional speech:

• Pega aquele trem ali. = Grab that thing there.

But in your sentence, because of the context with estação, trem clearly means train.

In Brazilian Portuguese, trem ends with a nasal sound.

A rough approximation is something like trehng or trẽj̃, but neither is exact in English.

A few points:

• tr is pronounced with a tapped r
• the e is like eh
• the final m is not fully pronounced like English m
• instead, it nasalizes the vowel before it

So the ending is nasal, not a strong m sound.

A simple Brazilian Portuguese approximation:

• está → es-TA
• estação → es-ta-SOWN
  (but with a nasal ending, not a normal English own)

A few details:

• á shows the stressed syllable in está
• ção is pronounced roughly like são with a nasal ão
• ç sounds like s

So:

• está has stress on the last syllable
• estação has stress on the final ção

Yes. Portuguese word order is flexible, although the original sentence is the most neutral and natural.

Possible variations:

• Na estação, o trem está parado.
• O trem está na estação, parado.

These can sound slightly different in emphasis, but they are understandable.

The original:

• O trem está parado na estação.

is the clearest standard way to say it.

Yes.

• O trem está na estação. = The train is at the station.

That only tells you the train’s location.

When you add parado, you also describe its condition:

• O trem está parado na estação. = The train is stopped at the station.

So parado adds the idea that the train is not moving.

Because o trem is already the subject.

Portuguese does not need an extra subject pronoun when the noun itself is present.

So:

• O trem está parado na estação.

is complete and natural.

You could say:

• Ele está parado na estação.

if ele refers back to o trem from an earlier sentence. But if you are introducing the subject, using the noun itself is better.