Quiero sentarme junto a la ventana en el tren.

Because sentarse means to sit down, while sentar usually means to seat someone/something or to make someone sit.

• Quiero sentarme = I want to sit down
• Quiero sentar al niño = I want to seat the child

So in your sentence, the speaker is doing the action to themself, which is why Spanish uses the reflexive form sentarse.

The -me is the reflexive pronoun meaning myself. In this sentence, it shows that the subject and the person affected by the action are the same.

• sentarme = to sit myself / to sit down
• sentarte = to sit yourself / to sit down
• sentarse = to sit oneself / to sit down

In natural English, we usually just say sit down, not sit myself, but Spanish often uses the reflexive structure here.

Yes, both are correct:

• Quiero sentarme
• Me quiero sentar

When you have a conjugated verb plus an infinitive, the reflexive pronoun can usually go:

1. attached to the infinitive
2. before the conjugated verb

So both versions mean the same thing. In this sentence, Quiero sentarme is very natural.

They describe different things:

• sentarse = to sit down → the action
• estar sentado = to be seated / to be sitting → the state or result

Examples:

• Quiero sentarme junto a la ventana.
  = I want to sit down by the window.

• Quiero estar sentado junto a la ventana.
  = I want to be seated by the window.

In many situations both are possible, but sentarme focuses on the act of taking the seat.

Yes, junto a means next to, beside, or by.

So:

• junto a la ventana = by the window / next to the window

It is a very natural expression in Spanish. Other common alternatives are:

• al lado de la ventana = next to the window
• cerca de la ventana = near the window

Of these, junto a and al lado de suggest being right beside it more strongly than cerca de.

Yes. Quiero sentarme al lado de la ventana en el tren is also correct and natural.

The difference is small:

• junto a sounds a bit more compact and sometimes slightly more formal or neutral
• al lado de is very common and everyday

Both would be understood perfectly in Spain.

Because the expression is junto a.

It works as a fixed combination:

• junto a la ventana
• junto al coche
• junto a mí

You should learn junto a as a unit.
By contrast:

• al lado de la ventana uses de
• en la ventana would mean in/on the window, which is a different idea

So junto a la ventana is the correct structure for next to the window.

Both are possible, but they mean slightly different things.

• la ventana = the window, usually meaning a specific seat by the window
• una ventana = a window, less specific

In real situations on a train, la ventana is very natural because people commonly talk about the window seat area in a definite way.

Compare:

• Quiero sentarme junto a la ventana.
  = I want to sit by the window.

• Quiero sentarme junto a una ventana.
  = I want to sit by a window.

The second one sounds more indefinite.

Because it adds extra information about where this whole situation is happening: on the train.

Spanish often places this kind of location phrase after the main idea:

• Quiero sentarme junto a la ventana en el tren.

This is natural word order. But other orders are also possible, depending on emphasis:

• En el tren, quiero sentarme junto a la ventana.
• Quiero sentarme en el tren junto a la ventana.

The original version is a normal, neutral way to say it.

Yes. In Spanish, transport is usually expressed with en:

• en el tren = on the train
• en el autobús = on the bus
• en el avión = on the plane

Spanish uses en where English often uses on.

Sobre el tren would mean something like on top of the train or about the train, so it is not correct here.

Quiero is grammatically fine, but yes, it can sound a bit direct depending on context.

More polite options are:

• Me gustaría sentarme junto a la ventana en el tren.
  = I would like to sit by the window on the train.

• Quisiera sentarme junto a la ventana en el tren.
  = I would like to sit by the window on the train.

If you are speaking to staff or making a request, those versions are often softer than quiero.

In normal use, it most naturally means I want a seat by the window on the train.

The phrase junto a la ventana strongly points to seat position.
And en el tren gives the setting.

So a listener would usually understand it as a preference for where you want to sit during the train journey.

You could say:

• Quiero sentarme junto a la ventana en un tren.

Here, un tren means a train, not a specific one.
Compare:

• en el tren = on the train, a specific train or the current one
• en un tren = on a train, more general or indefinite

So the article changes depending on whether the train is specific or not.