Plus le train avance, plus notre wagon se remplit, si bien que nous finissons par rester debout.

This is a very common French pattern: plus..., plus...

It means the more..., the more...

So:

• Plus le train avance = The farther / more the train goes on
• Plus notre wagon se remplit = the more our carriage fills up

This structure is called a correlative comparative. French often uses it exactly this way, with plus at the start of both clauses.

Other examples:

• Plus j’étudie, plus je comprends.
  The more I study, the more I understand.
• Plus il parle, plus je me fatigue.
  The more he talks, the more I get tired.

There is a verb there: avance.

The clause is built normally:

• le train = subject
• avance = verb

So Plus le train avance literally has the structure The more the train moves forward / progresses.

French does not need any extra word to build this pattern. You simply start the clause with plus, then continue with a normal sentence.

Here avance comes from avancer, which usually means:

• to move forward
• to advance
• to make progress

With a train, le train avance means the train is continuing along its route, moving onward. In this sentence, it suggests as the journey continues or as the train goes farther.

So it is not necessarily about the train speeding up. It is about the train progressing along the line.

In French, wagon can mean a railway car, carriage, or wagon, depending on context.

In everyday modern French, for passenger trains, people also very often say:

• voiture
• wagon

Both can refer to a train car, though usage can vary by region and context.

Important point for English speakers: French wagon is not restricted to the exact same uses as English wagon. In this sentence, notre wagon clearly means our train car / carriage.

Notre wagon means our carriage/train car. It identifies the specific car the speakers are in.

Using le wagon would simply mean the carriage, which is possible in some contexts, but notre makes it more personal and concrete: this is the car we are riding in.

So the sentence focuses on the speakers’ direct experience.

Because the verb here is se remplir, which means:

• to fill up
• to become full

This is a reflexive/pronominal use.

Compare:

• On remplit le wagon.
  People are filling the carriage.
  Here remplir is transitive: someone fills something.

• Le wagon se remplit.
  The carriage is filling up / becoming full.
  Here the carriage itself is presented as undergoing the process.

So se remplit is the natural way to say that the wagon is gradually becoming crowded.

Not exactly. It is closer to a middle or pronominal construction than a true passive.

• Le wagon est rempli = The carriage is filled / full
  This is more like a passive result or state.

• Le wagon se remplit = The carriage is filling up
  This emphasizes the process as it happens.

For an English speaker, it is often easiest to think of se remplir here as to fill up.

French often uses the present tense for:

• general truths
• habitual situations
• vivid narration

Here the present makes the scene feel immediate and alive:

• avance
• se remplit
• finissons

Even if the event happened in the past in real life, French can use the present to narrate it vividly. It can also describe what typically happens on a journey like this.

So the present tense here is very natural.

Si bien que is a connector of consequence. It means:

• so that
• with the result that
• so
• as a result

In this sentence, it introduces what happens because the carriage gets fuller and fuller.

So:

• si bien que nous finissons par rester debout
  = so / with the result that we end up having to remain standing

It is a slightly more formal or literary connector than plain donc.

All three can express consequence, but not in exactly the same way.

• donc = so, therefore
• alors = often so, then
• si bien que = so that / with the result that

Si bien que more strongly links the second clause as the natural result of the first. It can sound a bit more polished or narrative.

Compare:

• Le wagon se remplit, donc nous restons debout.
  Correct, but more straightforward.

• Le wagon se remplit, si bien que nous restons debout.
  More elegant, emphasizing the consequence.

Finir par + infinitive means:

• to end up doing something
• to finally do something
• to eventually do something

So:

• nous finissons par rester debout
  = we end up standing / we end up having to remain standing

This adds the idea that this is the final result after a process. First the train continues, then the wagon fills up, and eventually the consequence is that there are no seats left.

If you said simply nous restons debout, you would lose that sense of progression and outcome.

Both can be possible in French, but they are not exactly the same.

• être debout = to be standing
• rester debout = to remain standing / to stay standing

Here rester debout suggests that the speakers must continue standing instead of sitting down. It fits the idea that the carriage becomes so full that they cannot sit.

So rester debout is more natural because it emphasizes the ongoing state.

Debout means standing or upright.

In rester debout, it functions as a predicate complement after the verb rester. In simple learner terms, it describes the subject’s state.

Compare:

• Il est debout.
  He is standing.
• Nous restons debout.
  We remain standing.

So you can think of debout as describing how the subject is positioned.

Because the plus..., plus... pattern is a fixed structure.

When French means the more..., the more..., plus goes at the beginning of each clause:

• Plus X..., plus Y...

If you say le train avance plus, that usually means something else, such as:

• the train is moving more
• or in some contexts the train is no longer moving, because ne... plus means no longer

So starting each clause with plus is essential here to show the correlative meaning.

Yes, but the meaning and rhythm would be a little different.

For example:

• Au fur et à mesure que le train avance, notre wagon se remplit...

That means something like:

• As the train moves along, our carriage fills up...

This is perfectly possible, but plus..., plus... is especially good for expressing a rising relationship between two things:

• more progress of the train
• more crowding in the carriage

So the original sentence highlights that proportional increase very neatly.

The commas are natural because the sentence has three linked parts:

1. Plus le train avance
2. plus notre wagon se remplit
3. si bien que nous finissons par rester debout

French punctuation often separates these kinds of clauses clearly, especially in longer sentences. The commas help the reader follow the logical progression:

• first development
• second development
• consequence

Yes, very often. It is a useful everyday structure.

Examples:

• Il a fini par accepter.
  He eventually accepted.
• On finit par s’habituer.
  You end up getting used to it.
• Elle finit par comprendre.
  She finally understands / ends up understanding.

It often implies:

• some delay
• some process
• a final outcome after hesitation or change

That is exactly why it works well in this sentence.

A learner could remember these chunks:

• Plus..., plus... = the more..., the more...
• se remplir = to fill up
• si bien que = so that / with the result that
• finir par + infinitive = to end up doing
• rester debout = to remain standing

If you learn those as ready-made patterns, the whole sentence becomes much easier to understand and reuse.