Learn Hardcore Japanese: asa no densya ha manin desu.

の is a genitive/attributive particle that links two nouns. Here it turns 朝 (“morning”) into a modifier of 電車, so 朝の電車 literally means “the train of the morning,” i.e. “morning train.”

Why is は used after 電車 instead of が?

は marks the topic of the sentence. By saying 朝の電車は, you introduce “the morning train” as what you’re talking about and then comment on it. If you used が, you’d be simply stating that it exists/is full, often in response to a question like “Which train is full?”

What part of speech is 満員, and how does it function here?

満員 is a noun meaning “full (of people)” or “full capacity.” In this sentence it’s used as the predicate noun, paired with the copula です to mean “is full.”

Why is です added at the end? Can I drop it?

です is the polite copula equivalent to “is.” In casual speech you can replace it with the plain copula だ:
朝の電車は満員だ。
But you can’t just drop it entirely—Japanese needs either だ/です or a verb to complete the predicate.

Could you also say 朝の電車が満員です? What’s the nuance difference?

Yes. Using が makes “the morning train” the grammatical subject, whereas は makes it the topic.

が often appears when you’re pointing out a fact (“The morning train is full.”)
は is used when you’re making a general statement about that topic.

How is 満員 pronounced?

It’s read まんいん (man’in). The apostrophe in the romanization shows you need to pause slightly so ん doesn’t blend with the following vowel.

Are there other ways to say “the train is crowded” in Japanese?

Yes. A few common alternatives:
• 満員電車 – a compound noun meaning “crowded train”
• 混雑している – a verb phrase meaning “is congested/crowded,” e.g. 朝の電車は混雑しています。

Do Japanese sentences normally have spaces between words?

No. Standard Japanese writing doesn’t use spaces. They’re often inserted in learner materials to show word boundaries. Normally you’d write it as:
朝の電車は満員です。

Why is 朝の電車 placed at the beginning of the sentence?

Japanese often follows a [Time] + [Topic] + [Comment] order. Placing 朝の電車 first tells the listener “This is about the morning train,” then follows with the comment that it’s full.