ukkari nete simatte, oriru eki wo mata norisugosimasita.

うっかり means something like carelessly, accidentally, or without meaning to.

In this sentence, it shows that the speaker did not plan to fall asleep. It adds the nuance of a mistake caused by inattention:

• うっかり寝る = to accidentally fall asleep
• うっかり忘れる = to accidentally forget

So うっかり sets up the whole sentence as an unintentional mistake.

The pattern 〜てしまう often adds one of these nuances:

• something is completely done
• something happened unintentionally
• something happened and the speaker feels it was unfortunate or regrettable

So:

• 寝て = fell asleep / sleeping and...
• 寝てしまって = accidentally fell asleep / ended up falling asleep, and...

Here, しまう strongly fits the situation because missing your stop is an unwanted result.

Also, しまって is the て-form of しまう, so it connects to the next clause.

The て-form connects this part to what follows.

So the sentence flows like this:

• うっかり寝てしまって、
• 降りる駅をまた乗り過ごしました。

This means something like:

• I accidentally fell asleep, and as a result, I passed my station again.

In natural English, this often sounds causal, even if Japanese does not explicitly say because:

• Because I accidentally fell asleep, I missed my stop again.

This is a very common question.

降りる駅 means the station where I get off / the station I’m supposed to get off at.

In Japanese, a verb in plain form can directly modify a noun:

• 食べる人 = the person who eats / the person who will eat
• 行く場所 = the place I go / will go to
• 降りる駅 = the station where I get off

Here, 降りる is not really about past tense. It is describing the station by its function: the station to get off at.

If you said 降りた駅, that would mean the station where I got off—which does not fit, because the speaker actually did not get off there.

Because 降りる駅 is a relative clause.

In Japanese, when a verb modifies a noun, you usually do not put a particle like の between them:

• 読む本 = the book that I read / will read
• 住む町 = the town where I live
• 降りる駅 = the station where I get off

So 降りる directly modifies 駅.

Because 乗り過ごす means to ride past, to miss by staying on too long, or to go past one’s stop, and the place passed is marked with を.

So:

• 駅を乗り過ごす = to pass one’s station / miss one’s stop

This may feel strange because English often uses a preposition like past or at, but in Japanese, 乗り過ごす commonly takes the station as its object.

So 降りる駅を乗り過ごしました literally means something like:

• I rode past the station where I was supposed to get off.

乗り過ごす is made from:

• 乗る = to ride, get on
• 過ごす = to pass, let go by

Together, 乗り過ごす means:

• to stay on the train/bus too long
• to go past your stop
• to miss your stop by not getting off

It is more specific than just saying 降りませんでした (I didn’t get off).
乗り過ごしました tells you that the speaker remained on the train and went beyond the intended station.

また means again.

It shows this is not the first time the speaker has done this:

• また乗り過ごしました = I missed my stop again

It adds a slightly embarrassing or frustrated tone in this context, especially together with うっかり and 〜てしまう.

Yes. Japanese often omits the subject when it is understood from context.

A full version could be:

• 私は、うっかり寝てしまって、降りる駅をまた乗り過ごしました。

But in natural Japanese, 私は is usually unnecessary here.

The listener normally understands that the speaker is talking about themself.

The final verb is 乗り過ごしました, which is the polite past form.

• dictionary form: 乗り過ごす
• polite present/future: 乗り過ごします
• polite past: 乗り過ごしました

So the sentence is in polite style overall.
Even though earlier parts use plain forms like 降りる and 寝てしまって, that is normal because only the final verb determines the sentence’s politeness level in many cases.

You can break it down like this:

• うっかり = accidentally
• 寝てしまって = fell asleep unintentionally / unfortunately, and...
• 降りる駅 = the station to get off at
• を = marks that station as the object of 乗り過ごす
• また = again
• 乗り過ごしました = passed it / missed it by staying on

So the structure is:

• [Accidentally falling asleep] + [I again passed the station where I should get off].

A very literal order would be:

• Accidentally having fallen asleep, I again rode past the station to get off at.

Yes, in natural English that works very well.

Even though oversleep is not a word-for-word match for each piece of the Japanese, it captures the idea nicely:

• the speaker fell asleep
• because of that, they missed their stop
• it happened again

A more literal translation would be:

• I accidentally fell asleep and passed the station where I was supposed to get off again.

A more natural English translation could be:

• I accidentally fell asleep and missed my stop again.
• I overslept my stop again.

Not always, but it often suggests some kind of emotional coloring.

With 〜てしまう, the feeling depends on context:

• 宿題をしてしまった。
  = I finished my homework.
  This may just mean completion.

• 財布をなくしてしまった。
  = I lost my wallet.
  This clearly sounds regrettable.

In this sentence, since missing your station is inconvenient, 寝てしまって sounds regretful or apologetic: I went and fell asleep / I accidentally fell asleep.

Yes. The plain form before a noun does not by itself mark tense the same way English does.

So 降りる駅 can mean:

• the station where one gets off
• the station one will get off at
• the station one is supposed to get off at

In this sentence, context makes it mean:

• the station I was supposed to get off at

That is why the plain form works so naturally here.