In dem Abteil, auf das wir uns zuerst gefreut hatten, war es viel lauter, als wir erwartet hatten.

The core main clause is war es viel lauter.

Everything else attaches to that core:

• In dem Abteil = a prepositional phrase at the beginning
• auf das wir uns zuerst gefreut hatten = a relative clause describing Abteil
• als wir erwartet hatten = a comparison clause after lauter

So the sentence is basically:

• In dem Abteil ... war es viel lauter ...
• with extra information inserted to specify which compartment and compared to what expectation

Here Abteil means a compartment, especially in a train.

Depending on context, Abteil can also mean a section or partitioned area, but in this sentence the natural meaning is a train compartment.

Im is just the contraction of in dem.

So grammatically, im Abteil and in dem Abteil come from the same thing. Here, the full form in dem is very natural because:

• it sounds a little clearer before the relative clause
• it can give slightly more emphasis to that particular compartment

So in dem Abteil, auf das ... sounds very natural. A contraction would not be impossible, but the full form works especially well here.

Because the verb is sich freuen auf when it means to look forward to.

So the important pattern is:

• Wir freuen uns auf das Abteil.
• In a relative clause: das Abteil, auf das wir uns gefreut hatten

The preposition auf belongs to the verb sich freuen auf, not to Abteil by itself.

Because sich freuen auf takes the accusative.

The noun being referred to is das Abteil, which is:

• singular
• neuter

The accusative form of the relative pronoun for a neuter singular noun is das.

So:

• nominative: das
• accusative: das
• dative: dem

Since auf here is part of sich freuen auf and requires the accusative, German uses auf das.

Because sich freuen is a reflexive verb.

With wir, the reflexive pronoun is uns:

• wir freuen uns
• wir hatten uns gefreut
• auf das wir uns gefreut hatten

So uns is not optional here. It is part of the verb construction.

Because German main clauses follow the verb-second rule.

The first position is already occupied by the opening phrase:

• In dem Abteil, auf das wir uns zuerst gefreut hatten

So the finite verb must come next:

• war

Then the subject or subject-like element comes after it:

• es

That is why you get:

• In dem Abteil ... war es viel lauter

and not:

• In dem Abteil ... es war viel lauter

Because both actions happened before the past situation described by war.

Timeline:

• first, we looked forward to the compartment
• first, we expected something about it
• later, in that compartment, it was much louder

So German uses Plusquamperfekt:

• wir uns ... gefreut hatten
• wir erwartet hatten

This is the same logic as English had looked forward and had expected.

Because both are in subordinate clauses:

• auf das wir uns zuerst gefreut hatten = relative clause
• als wir erwartet hatten = comparison clause

In German subordinate clauses, the finite verb goes to the end. With a compound tense, the usual order is:

• participle first
• auxiliary last

So:

• main clause: Wir hatten uns gefreut
• subordinate clause: ..., auf das wir uns gefreut hatten

The same thing happens with erwartet hatten.

Zuerst means at first, initially, or to begin with.

It suggests that their first reaction was positive: they were initially pleased about that compartment, before actually experiencing how noisy it was.

Lauter is the comparative of laut:

• laut = loud
• lauter = louder

Viel intensifies the comparative, so viel lauter means much louder or a lot louder.

Also, there is no adjective ending on lauter here because it is used predicatively after sein:

• Es war laut
• Es war lauter
• Es war viel lauter

Here als means than because it follows a comparative:

• viel lauter, als wir erwartet hatten = much louder than we had expected

This is the normal pattern in German:

• größer als = bigger than
• schneller als = faster than
• lauter als = louder than

This is different from wie, which is used for equality:

• so laut wie = as loud as

Als can also mean when for a single event in the past, but not in this sentence.

Here it clearly means than, because it comes after the comparative viel lauter.

So in this sentence:

• als = than

not:

• when

Here es is an impersonal or general it, like in English:

• It was much louder

It refers to the overall situation or noise level in the compartment, not to a specific object in a strict grammatical sense.

So it does not mean that es is standing in for Abteil because of gender. It is just the normal way German says that the environment or situation was loud.

In standard German, when you have a clear noun antecedent like das Abteil, the usual and preferred form is:

• das Abteil, auf das wir uns gefreut hatten

Using worauf is much more natural when there is no specific noun antecedent, for example:

• Worauf habt ihr euch gefreut?

So in this sentence, auf das is the better and more standard choice.

There are commas for two reasons:

• auf das wir uns zuerst gefreut hatten is a relative clause, and German sets relative clauses off with commas
• als wir erwartet hatten is a subordinate comparison clause, and that also takes a comma

So the commas are required by normal German punctuation rules.