Le train direct s’arrête à cause de la panne.

Le means the, while un means a or an.

So le train direct refers to a specific train: the direct train.
If you said un train direct, it would mean a direct train, with a less specific sense.

French often uses the definite article when the speaker has a particular thing in mind, just as English does.

In French, many adjectives come after the noun, unlike in English.

So:

• un train direct = a direct train
• une panne grave = a serious breakdown

Some adjectives do come before the noun, but direct normally comes after it here.

Train direct means a train that goes directly to its destination or makes few or no intermediate stops.

Depending on context, English might translate it as:

• direct train
• express train
• through train

It is not always a perfect one-to-one match with express, but the basic idea is that it is not making the usual local stops.

The verb here is s’arrêter, which means to stop or to come to a stop.

So:

• Le train s’arrête = The train stops
• Le conducteur arrête le train = The driver stops the train

This is an important difference:

• arrêter something = to stop something
• s’arrêter = to stop oneself / to come to a stop

In French, many verbs change meaning when used reflexively, and this is one of them.

Yes, historically and grammatically it is reflexive, so it can be understood that way. But in normal modern French, you should usually just translate it naturally as to stop.

So Le train s’arrête is best understood as:

• The train stops
• The train is stopping

—not awkwardly as the train stops itself, unless you specifically want to emphasize the reflexive idea.

The apostrophe appears because se becomes s’ before a vowel sound.

So:

• se + arrête → s’arrête

This is very common in French:

• je aime → j’aime
• ne est pas → n’est pas
• se appelle → s’appelle

It helps the sentence sound smoother.

The circumflex in arrête is part of the spelling of this verb form.

Here, arrête is the third-person singular present form of s’arrêter:

• je m’arrête
• tu t’arrêtes
• il / elle / on s’arrête

The accent helps distinguish pronunciation and spelling. In many -e + consonant + e patterns, the middle vowel changes in forms like this:

• lever → je lève
• acheter → j’achète
• s’arrêter → je m’arrête

So the accent is not optional here.

French often uses the present tense in places where English also uses the present:

• Le train s’arrête = The train stops / The train is stopping

Depending on context, the French present can describe:

• something happening now
• a general fact
• a scheduled or regular event

So this sentence could describe what is happening right now, or what happens in this situation.

À cause de means because of and is followed by a noun.

Here, the cause is a noun phrase:

• la panne = the breakdown / the malfunction

So:

• à cause de la panne = because of the breakdown

This is different from parce que, which is usually followed by a clause with a verb:

• Le train s’arrête parce qu’il y a une panne. = The train stops because there is a breakdown.

So a simple way to remember it is:

• à cause de + noun
• parce que + verb clause

Because à cause de must be followed by de, and then the noun keeps its article.

Since panne is feminine singular, it takes la:

• à cause de la panne

Compare:

• à cause du retard = because of the delay
  (de + le = du)
• à cause de l’accident = because of the accident
• à cause des grèves = because of the strikes
  (de + les = des)

So de la is not unusual here—it is just de plus the feminine article la.

Une panne usually means a breakdown, failure, or malfunction.

It is commonly used for machines, systems, electricity, vehicles, and similar things. For example:

• une panne de courant = a power outage
• une panne de moteur = engine failure
• être en panne = to be broken down / out of order

In this sentence, la panne suggests some technical problem causing the train to stop.

You could say it, but it means something a bit different.

• Le train direct s’arrête = The direct train stops / is stopping
• Le train direct est arrêté = The direct train is stopped

The first focuses on the action of coming to a stop.
The second focuses on the resulting state: it is already stopped.

So in your sentence, s’arrête is the more natural choice if the idea is that the train comes to a halt because of the breakdown.