Я зевала в электричке и поняла, что ночью опять не смогла выспаться.

Not quite. Электричка usually means a suburban / commuter electric train, the kind people take between a city and nearby towns or suburbs.

So в электричке is more specific than just в поезде (on the train / in a train). A Russian learner will hear электричка very often in everyday speech.

Because after в when it means in / on / inside a place, Russian uses the prepositional case.

• электричка → dictionary form, nominative
• в электричке → prepositional singular

So:

• в электричке = on the commuter train / in the commuter train

This is the normal way to say location here.

Because зевала is the imperfective past form of зевать (to yawn), and it suggests an ongoing or repeated action.

• зевала = was yawning / yawned repeatedly
• зевнула = gave one yawn

In this sentence, the idea is that she was sitting on the train, yawning, and then realized something. So the imperfective fits better.

Because these are past tense feminine singular forms.

In Russian past tense, verbs agree with the subject in gender and number:

• masculine: зевал, понял, смог
• feminine: зевала, поняла, смогла
• neuter: зевало, поняло, смогло
• plural: зевали, поняли, смогли

So this sentence tells us the speaker is female.

If the speaker were male, it would be:

Я зевал в электричке и понял, что ночью опять не смог выспаться.

Because что introduces a subordinate clause:

• Я поняла = I realized
• что ночью опять не смогла выспаться = that I again didn’t manage to get enough sleep at night

In Russian, subordinate clauses introduced by что are normally separated by a comma.

Ночью is a very common adverbial form meaning at night / during the night.

It comes from ночь, but here it is used in a fixed time expression. Russian often uses forms like this to express time:

• утром = in the morning
• днём = during the day / in the afternoon
• вечером = in the evening
• ночью = at night

So ночью опять не смогла выспаться means again didn’t manage to get enough sleep during the night.

Опять means again.

In many contexts, опять and снова are close in meaning. But опять very often carries the feeling of yet again, sometimes with mild annoyance or frustration.

So here:

ночью опять не смогла выспаться
= I again / yet again couldn’t get enough sleep at night

That fits the context well: the speaker realizes this is not the first time.

Выспаться means to get enough sleep, to sleep one’s fill, or to sleep properly so that you feel rested.

It is not just to sleep. It means sleep with the result of being sufficiently rested.

Compare:

• спать = to sleep
• поспать = to sleep for a while
• выспаться = to get enough sleep / sleep enough

So:

• Я не выспалась = I didn’t get enough sleep
• Мне нужно выспаться = I need to get some proper sleep

Both are possible, but they are not exactly the same.

• не выспалась = didn’t get enough sleep
• не смогла выспаться = wasn’t able to get enough sleep / couldn’t manage to get enough sleep

The version with смочь adds a sense of inability or failure to achieve the result. It can sound a little stronger, as if circumstances prevented it.

So the sentence suggests something like:

I realized that once again, during the night, I hadn’t managed to get enough sleep.

Because it depends on смогла.

смочь means to be able / to manage, and it is followed by an infinitive:

• смогла выспаться = managed to get enough sleep
• не смогла выспаться = didn’t manage to get enough sleep

This is very similar to English could / managed to + verb.

Because выспаться is a perfective verb that focuses on a completed result: becoming well-rested.

That makes sense after смочь, because the meaning is about whether the result was achieved:

• смогла выспаться = succeeded in getting enough sleep
• не смогла выспаться = failed to achieve that result

If Russian wants to talk about the process of sleeping in general, it would use a different verb, such as спать.

Yes, выспаться is a reflexive verb because it ends in -ся.

But here -ся does not mean a literal oneself in the English sense. It is just part of the verb’s normal form. You should learn выспаться as a whole vocabulary item meaning to get enough sleep.

Many Russian verbs with -ся have meanings that are not translated with oneself in English.

Yes, Russian word order is fairly flexible. This version sounds natural and neutral.

The sentence puts the time expression first inside the subordinate clause:

• ночью = at night
• опять = again
• не смогла выспаться = couldn’t get enough sleep

Other orders are possible, for example:

• что опять ночью не смогла выспаться
• что не смогла опять выспаться ночью

But they may shift emphasis slightly. The original version sounds smooth and everyday.

Here поняла is best translated as realized.

Although понять often means to understand, in contexts like this it commonly means to realize / become aware of something:

• Я поняла, что устала. = I realized that I was tired.
• Он понял, что ошибся. = He realized that he had made a mistake.

So in this sentence, the speaker was yawning and then realized the reason.

Yes, that is a very good translation.

Because зевала is imperfective, was yawning often works especially well in English. Depending on context, I yawned on the commuter train is also possible, but I was yawning better reflects the ongoing sense of the Russian.