Learn Hardcore Japanese: densya de ha, itumo tuurogawa yori madogawa no seki wo erabu you ni site iru. - On trains I always try to choose a window seat rather than an aisle seat.

Here は is slightly contrastive: it suggests on trains (as opposed to other situations) I tend to choose window seats. You could imagine a contrast like:

電車では窓側の席を選ぶようにしているが、バスでは通路側を選ぶ。
→ On trains I try to choose a window seat, but on buses I choose an aisle seat.

Without は (電車で、いつも…) it would still be grammatical, but it would sound more like a simple description without that gentle contrastive or “as for…” nuance.

Why is で used after 電車 instead of に?

で and に can both be used with locations, but they have different roles:

で = place where an action is performed
に = place of existence or destination

Here, 席を選ぶ (choose a seat) is an action taking place in/on the train, so we use 電車で.

Compare:

電車で本を読む。
→ I read books on the train.
電車に乗る。
→ I get on the train (train is the destination).

So 電車で席を選ぶ fits the pattern “do action X at/in place Y,” which takes で.

What does 通路側より窓側 literally mean, and why is より used?

Literally:

通路側 = aisle side (aisle seat)
窓側 = window side (window seat)
A より B = B rather than A / B more than A

So 通路側より窓側 is “window side rather than aisle side.”

The structure is:

[A] より [B] → (I prefer / choose) B rather than A

The preferred or chosen item comes after より. Here, the speaker prefers 窓側 (window side), so it is placed after より.

Why doesn’t 通路側 have a particle after it before より?

When より is used for comparison, it directly follows the thing being compared, without another particle:

東京より大阪のほうが暑い。
→ Osaka is hotter than Tokyo.
コーヒーより紅茶が好きだ。
→ I like tea rather than coffee.

Likewise:

通路側より窓側の席
→ a window seat rather than an aisle seat

If you tried to add a particle like は or が before より, it would either sound unnatural or change the structure. In simple comparisons like this, the noun goes straight into より.

Why is it 窓側の席 instead of just 窓側席?

Japanese usually links nouns with の when one noun modifies another:

窓側の席
→ a seat on the window side (window seat)
学校の友だち
→ my friend from school

Compounds like 窓側席 do exist, but they are more technical, shorter forms often seen in timetables, booking systems, or announcements. In normal, natural spoken or written Japanese, 窓側の席 is much more common and sounds smoother and more colloquial.

What does 〜ようにしている mean here, and how is it different from just 選んでいる?

Verb-dictionary-form + ようにしている means:

“I make it a point to do X.”
“I try to do X (as a habit/rule).”

It suggests a conscious effort or a personal rule.

So:

選ぶようにしている
→ I make sure to choose / I try to choose

If you said:

窓側の席を選んでいる。

it would sound more like a neutral description of a current ongoing habit:
“I (typically) choose window seats.”

Nuance difference:

選んでいる = describes what usually happens.
選ぶようにしている = describes what you try or make an effort to do, your policy.

In English, 選ぶようにしている is closer to “I always try to choose” or “I make a point of choosing.”

What is the role of いつも in this sentence, and where can it appear?

いつも means “always / usually.” In the sentence, it modifies the whole action of choosing window seats:

電車では、いつも 通路側より窓側の席を選ぶようにしている。
→ On trains, I always try to choose a window seat rather than an aisle seat.

It can move a bit within the clause as long as it stays near the verb phrase:

電車では、通路側より窓側の席をいつも選ぶようにしている。
電車では、通路側よりいつも窓側の席を選ぶようにしている。

All are understandable, but the original position (after 電車では) is very natural and clear, signaling that “always” applies to the whole behaviour on trains.

Who is the subject of this sentence? There is no 私 written.

In Japanese, the subject (often 私 = I) is frequently omitted when it is clear from context.

Here the action is about a personal preference for seats, so the natural understood subject is:

(私は) 電車では、いつも通路側より窓側の席を選ぶようにしている。

In English we must say “I,” but in Japanese it is very common and more natural to leave 私 out if it’s obvious who is being talked about.

Could this sentence be rephrased with ほうが好き instead of 席を選ぶようにしている?

Yes, but the nuance changes:

電車では、通路側より窓側の席のほうが好きだ。
→ On trains, I like window seats more than aisle seats.

This version states a preference.

The original:

…窓側の席を選ぶようにしている。

states a habitual action / personal rule (what the speaker actually does or tries to do). So:

のほうが好きだ = I prefer it (in feeling).
選ぶようにしている = I choose it as a habit / I make a point of choosing it.

Both can be true, but they emphasize different things.

Is it okay to drop は and just say 電車で、いつも…? What would change?

Grammatically, you can say:

電車で、いつも通路側より窓側の席を選ぶようにしている。

This is fine and understandable.

However, without は, you lose the “as for on trains” or contrastive nuance. With 電車では, it feels more like:

“When it comes to trains in particular, I (always) try to choose a window seat.”

Without は, it is a more neutral statement, just describing what you do on trains, without subtly inviting a comparison to other situations.