hon wo yonde itara nieki mo norisugosite, siranai kaisatuguti kara dete simaimasita.

を marks the direct object of the verb.

• 本 = book
• 読む = to read
• so 本を読む = to read a book

In other words, 本 is the thing being read.

Here, 読んでいたら is better understood as while I was reading or when I was reading.

It comes from:

• 読む → to read
• 読んでいる → be reading
• 読んでいた → was reading
• 読んでいたら → while/when I was reading, ... / as I was reading, ...

Although たら often means if, in real sentences like this it very often means:

• when
• while
• and then, as a result

So this is not really a hypothetical if. It is describing what happened during that situation.

Because 〜ていたら connects the first situation to the unexpected result that follows.

So:

• 本を読んでいた = I was reading a book
• 本を読んでいたら、二駅も乗り過ごして... = While I was reading a book, I ended up missing my stop by two stations...

The 〜たら adds the sense that something happened as a consequence or discovery during that ongoing action. It often feels a little like:

• while I was doing X, Y happened
• I was doing X, and before I knew it, Y had happened

二駅 means two stations.

The も adds emphasis. It suggests something like:

• as many as two stations
• two whole stations
• even two stations

So 二駅も乗り過ごして sounds stronger than just 二駅乗り過ごして. It implies the speaker feels that overshooting by two stations was more than expected, annoying, or surprising.

乗り過ごす is a compound verb:

• 乗る = to ride
• 過ごす = to pass, let pass

Together, 乗り過ごす means:

• to ride past your stop
• to miss your stop by staying on the train/bus too long

So 二駅も乗り過ごして means I rode past my stop by two stations.

The て form, 乗り過ごして, connects this action to the next one:

• I overshot by two stations, and then
• I got out through an unfamiliar ticket gate

Because 知らない改札口 is a noun-modifying phrase, and Japanese normally uses the plain form before a noun.

• 知らない改札口 = a ticket gate / exit gate that I do not know, or am not familiar with

In natural English, this often becomes:

• an unfamiliar ticket gate
• a gate I didn’t know

Even though the whole sentence is in the past, Japanese often uses the non-past plain form in front of nouns like this.

So 知らない改札口 does not mean the time is present. It is simply the normal form used to describe the noun.

改札口 is the station gate area where you pass through the ticket barriers.

Depending on context, it can be translated as:

• ticket gate
• fare gate
• station exit gate

So 知らない改札口から出て means the speaker came out through a gate or exit at the station that was unfamiliar to them.

Because 出る often uses から to mark the place you come out of.

• 部屋から出る = leave the room
• 駅から出る = come out of the station
• 改札口から出る = go out through the ticket gate

So 改札口から marks the point of exit.

In English we might say through or from, but in Japanese から is the normal choice here.

〜てしまう adds an extra nuance. It often means one of these:

• something was completed fully
• something happened unintentionally
• the speaker feels regret, annoyance, or a slight sense of oops

So:

• 出ました = I went out
• 出てしまいました = I ended up going out / I went out and it was kind of an unintended or regrettable result

In this sentence, しまいました fits very well because the whole situation is a small mishap:

• the speaker got absorbed in reading
• missed their stop by two stations
• then ended up exiting through an unfamiliar gate

So 出てしまいました gives it that oops, that happened feeling.

This is very normal in Japanese.

Only the final verb of the sentence usually carries the politeness level for the whole sentence. Earlier verbs in subordinate clauses usually stay in plain form.

So in this sentence:

• 読んでいたら = plain subordinate form
• 乗り過ごして = plain connective form
• 出てしまいました = polite final verb

That is completely natural. You do not need to make every verb polite.

The subject is omitted, which is extremely common in Japanese.

From context, we naturally understand that the speaker is talking about themself:

• I was reading a book
• I missed my stop by two stations
• I ended up going out through an unfamiliar gate

Japanese often leaves out words like I, you, or he/she when they are obvious from context.

Yes. The flow is:

1. 本を読んでいたら
   While I was reading a book...

2. 二駅も乗り過ごして
   I ended up riding past my stop by two whole stations...

3. 知らない改札口から出てしまいました
   and ended up going out through an unfamiliar ticket gate.

So grammatically, the sentence links one situation to the next, and the overall feeling is that one thing led to another by accident.