Hon tar helst tåget när bussen är full av trängsel och buller.

Helst means preferably, rather, or by preference.

So Hon tar helst tåget means that taking the train is her preferred choice.

It often appears after the finite verb:

• Jag stannar helst hemma.
• Vi reser helst på hösten.

In Swedish, ta tåget is a very common expression meaning take the train.

So:

• ta tåget
• ta bussen
• ta bilen

You can also use åka tåg, but that means more travel by train than choose the train as your option.

So these are both possible, but they are not exactly identical:

• Hon tar helst tåget = she prefers to take the train
• Hon åker helst tåg = she prefers traveling by train

The version with tar tåget is especially natural when comparing transport options.

Swedish often uses the definite form with means of transport in common expressions like:

• ta tåget
• ta bussen
• ta bilen

This is just the normal idiomatic pattern.

English does not always match this exactly, so a native English speaker may expect something more like a bare noun, but Swedish usually wants the definite form here.

In bussen, the definite form also makes sense because it refers to the bus in that situation, or the relevant bus option in general.

Because bussen is an en-word.

Predicate adjectives agree like this:

• en-buss → bussen är full
• ett-tåg → tåget är fullt
• plural → bussarna är fulla

So the form full matches bussen correctly.

Full av means full of.

It is used with both concrete and abstract nouns:

• full av folk
• full av energi
• full av trängsel och buller

So bussen är full av trängsel och buller means the bus is filled with crowding and noise.

Because trängsel and buller are abstract or mass nouns here.

Swedish often uses such nouns without an article in expressions like this, just as English says:

• full of noise not
• full of a noise

If you used a definite form, it would usually refer to some specific crowding or noise already mentioned:

• det där bullret
• den där trängseln

Trängsel means crowding, congestion, or the feeling of too many people packed into a space.

Related words:

• trång = cramped, narrow
• trängas = to squeeze together, jostle

So full av trängsel suggests that the bus is uncomfortably crowded, not just that there are many people.

Buller means noise, especially loud background noise, din, or racket.

It is a normal Swedish word and sounds natural here. It is often used for environmental or ongoing noise, for example:

• trafikbuller
• buller från gatan

So in this sentence, buller fits well because it suggests an unpleasant noisy environment.

När means when or whenever, while om usually means if.

Here, när suggests a regular or real situation:

• she prefers the train when/whenever the bus is crowded and noisy

If you used om, it would sound more conditional:

• she prefers the train if the bus is crowded and noisy

Both can be possible in some contexts, but när is a very natural choice when talking about a recurring situation or a general preference.

Because när introduces a subordinate clause.

In Swedish, subordinate clauses normally keep subject + verb order:

• bussen är full = main clause
• när bussen är full = subordinate clause

So there is no inversion after när.

This is an important difference from main clauses in Swedish, where the verb often comes second.

In this sentence, helst comes after the finite verb and before the object:

• Hon tar helst tåget

That is a very common placement.

Compare:

• Jag dricker helst kaffe.
• Vi går helst till fots.
• Hon läser helst hemma.

So if you are learning Swedish word order, verb + helst + object/complement is a very useful pattern to remember.

Most naturally, it describes a habit or general preference.

Swedish present tense is often used for things that happen regularly or are generally true. So the sentence most likely means that she usually prefers the train whenever the bus is crowded and noisy.

Without more context, it does not strongly sound like a one-time event.