В этом вагоне слишком душно.

Because after в when you mean in/inside somewhere, Russian normally uses the prepositional case.

• вагон = nominative
• в вагоне = in the carriage
• этот вагон = this carriage
• в этом вагоне = in this carriage

So в этом вагоне means in this carriage/car, not this carriage as a subject.

It changes because it is in the prepositional case after в with a location meaning.

For a masculine noun like вагон, the prepositional singular often ends in -е:

• вагон → в вагоне
• дом → в доме
• музей → в музее

So вагоне is just the correct case form meaning in the carriage.

Этом is the prepositional singular masculine/neuter form of этот (this).

It agrees with вагоне, which is:

• singular
• masculine
• prepositional

So:

• nominative: этот вагон
• prepositional: в этом вагоне

Russian adjectives and pronouns usually have to match the noun in gender, number, and case.

Вагон usually means a railway car / train carriage / train car. Depending on context, it can also be used for a subway/metro car or sometimes other kinds of rail cars.

So in this sentence, it most naturally means this train car / carriage.

Душно means something like:

• stuffy
• airless
• close
• oppressively warm because there isn’t enough fresh air

It is not exactly the same as hot.

Compare:

• жарко = hot
• тепло = warm
• душно = stuffy / hard to breathe because the air feels heavy

A room, bus, or train car can be душно even if it is not extremely hot.

In this sentence, душно is not acting like a normal adjective modifying a noun. It is a predicative word or category-of-state word. In simpler learner terms, it is a word used to describe a general condition:

• Здесь холодно. = It’s cold here.
• В комнате тихо. = It’s quiet in the room.
• В этом вагоне душно. = It’s stuffy in this carriage.

So душно does not agree with a noun the way an adjective would. You do not say душный here, because the sentence is not the carriage is a stuffy one in an adjective sense; it is it is stuffy in the carriage.

In the present tense, Russian usually omits the verb “to be”.

So where English says:

• It is stuffy
• This carriage is too stuffy

Russian often just says:

• Душно.
• В этом вагоне слишком душно.

There is no need for a present-tense есть here. Using есть would sound wrong in this sentence.

Слишком means too, in the sense of more than is comfortable, acceptable, or desirable.

• очень душно = very stuffy
• слишком душно = too stuffy

So:

• В этом вагоне очень душно. = It’s very stuffy in this carriage.
• В этом вагоне слишком душно. = It’s too stuffy in this carriage.

Use слишком when you want to suggest a problem.

Yes. Russian word order is flexible, though the emphasis changes.

Neutral order:

• В этом вагоне слишком душно.

Other possible orders:

• Слишком душно в этом вагоне.
• Душно в этом вагоне.
• В этом вагоне душно.

These all mean roughly the same thing, but the focus shifts:

• В этом вагоне... puts the location first
• Слишком душно... emphasizes too stuffy
• Душно... starts with the condition itself

For learners, the original order is a very natural, neutral choice.

The stress is:

• в э́том ваго́не сли́шком ду́шно

A rough pronunciation guide:

• в Э-təm va-GO-nye SLISH-kəm DOOSH-nə

A few details:

• этом has stress on the first syllable: э́том
• вагоне has stress on -го-: ваго́не
• слишком has stress on the first syllable: сли́шком
• душно has stress on the first syllable: ду́шно

The о in unstressed syllables is reduced in normal speech.

Yes, absolutely.

• Здесь слишком душно. = It’s too stuffy here.
• В этом вагоне слишком душно. = It’s too stuffy in this carriage.

The second version is more specific. If everyone already knows you are talking about the carriage you are in, здесь may be enough.

Yes, it sounds natural. A Russian speaker might say it if:

• the train car is crowded
• there is no fresh air
• the windows are closed
• the ventilation is poor

It sounds like a normal complaint or observation. You might also hear similar sentences such as:

• В вагоне душно. = It’s stuffy in the carriage.
• Здесь душно. = It’s stuffy here.
• В этом вагоне очень душно. = It’s very stuffy in this carriage.

You probably can, but it means something a little differently and is less natural in this situation.

• В этом вагоне слишком душно = It is too stuffy in this carriage.
  This is the most natural way to describe the air/atmosphere.

• Этот вагон слишком душный = This carriage is too stuffy.
  Grammatically possible, but it sounds more like you are describing the carriage as an object with a property, not simply the current air condition.

In everyday speech, Russian usually prefers the impersonal version with душно for this kind of idea.