asa no densya ga tiensuru to, eki no hoomu ga hito de ippai ni narimasu.

It means morning trains or trains in the morning.

A few points:

• 朝 = morning
• の connects nouns, so 朝の電車 literally means morning’s trains or trains of the morning
• Japanese often does not mark singular vs. plural clearly, so this can refer to:
  • one morning train in a specific context, or
  • morning trains in general

In this sentence, it sounds most natural as a general statement, so morning trains is the best reading.

の connects one noun to another and shows a relationship between them.

In this sentence:

• 朝の電車 = trains of the morning / morning trains
• 駅のホーム = the platform of the station / the station platform

You can think of の as doing jobs like:

• of
• ’s
• an adjective-like connector between nouns

So の is very common for turning one noun into something that describes another noun.

Because there are really two clauses, and each clause has its own subject.

1. 朝の電車が遅延すると

   • subject: 朝の電車
   • が marks the trains as the thing that gets delayed

2. 駅のホームが人でいっぱいになります

   • subject: 駅のホーム
   • が marks the platform as the thing that becomes crowded/full

So the structure is basically:

• When morning trains are delayed,
• the station platform becomes full of people.

Each part has its own subject, so each part can have its own が.

遅延する means to be delayed or to experience a delay.

It is similar to 遅れる, but the nuance is different:

• 遅れる = ordinary, everyday to be late / to be delayed
• 遅延する = more formal, technical, or announcement-style to be delayed

For trains, both can appear, but:

• 電車が遅れる sounds more everyday and conversational
• 電車が遅延する sounds more formal, like news, notices, or railway announcements

Also, 遅延する is a suru-verb:

• 遅延 = delay
• する = to do

So literally it is something like to undergo delay.

No. Here と does not mean and.

In this sentence, と is a conditional meaning something like:

• when
• if
• whenever

So:

• 朝の電車が遅延すると、駅のホームが人でいっぱいになります。
• When morning trains are delayed, the station platform becomes full of people.

This と is often used when one thing naturally leads to another, especially as a regular result.

So the feeling is:

• If/when X happens, Y naturally happens.

Because this sentence describes a natural, regular consequence.

• A train delay happens
• then the platform fills up with people

That makes と a very good fit.

Roughly:

• と = when/if X happens, Y automatically or naturally follows
• たら = if/when X happens, then Y; more flexible and common in many situations
• なら = if it’s the case that X / as for X, then Y

In this sentence, と sounds good because it expresses a predictable result:

• When morning trains are delayed, the platform gets packed.

It sounds like a general fact or pattern, not a one-time event.

人でいっぱい means full of people.

Here, で marks what something is filled with.

So:

• 人でいっぱい = full with people = full of people

This pattern is very common:

• 水でいっぱい = full of water
• 荷物でいっぱい = full of luggage
• 人でいっぱい = full of people

If you used が, it would not express the same idea naturally here.
The point is not that people are the subject. The point is that the platform is filled with people, so 人 is the content filling it, and で marks that.

This is the pattern:

• A に なる = to become A

Here, いっぱい acts like a na-adjective / adjectival noun in this structure.

So:

• いっぱいだ = is full
• いっぱいになる = becomes full

In the sentence:

• 駅のホームが人でいっぱいになります
• The station platform becomes full of people

The に appears because なる often takes:

• a noun + に
• a na-adjective + に

Examples:

• 静かになる = become quiet
• 元気になる = become well / energetic
• いっぱいになる = become full

Because the sentence is describing a change of state.

• です = is
• なります = becomes

So:

• ホームが人でいっぱいです = the platform is full of people
• ホームが人でいっぱいになります = the platform becomes full of people

Since the idea is that the delay causes the platform to fill up, なる is the natural choice.

ホーム here means platform, specifically a train-station platform.

This is a very common Japanese railway word. It comes from an older borrowing related to platform usage, and in modern Japanese ホーム is the normal everyday word for a station platform.

So:

• 駅のホーム = station platform

Even though English speakers may expect platform, Japanese normally says ホーム.

Usually it sounds like a general statement, unless context makes it specific.

Japanese often leaves that kind of detail unstated. So this could mean:

• When morning trains are delayed, station platforms get crowded.
• or, in a specific context, the station platform gets crowded.

Because there is no article system like a / the in Japanese, you rely on context.

In isolation, the sentence feels like a general observation about what typically happens.

Yes, は could be used in some versions, but が works well here because each clause is presenting what happens.

Current sentence:

• 朝の電車が遅延すると、駅のホームが人でいっぱいになります。

This feels like a neutral description:

• When morning trains are delayed, the platform becomes full of people.

If you changed things to は, the nuance would shift more toward topic or contrast. For example:

• 朝の電車が遅延すると、駅のホームは人でいっぱいになります。

This can sound like:

• As for the station platform, it becomes full of people
• or slightly contrastive, depending on context

So the original with が is very natural for straightforward explanation.

Yes, that is very natural.

Your version means something like:

• When trains are delayed in the morning, the station platform becomes full of people.

Compared with the original:

• 朝の電車 focuses on morning trains
• 朝、電車が遅れると focuses on the timing: in the morning, when trains are delayed

Also:

• 遅れる is more everyday
• 遅延する is more formal

So both are fine, but they sound slightly different in style and focus.

It is:

[Clause 1] と、[Clause 2]

More specifically:

• 朝の電車が遅延すると

  • when morning trains are delayed

• 駅のホームが人でいっぱいになります

  • the station platform becomes full of people

So the full structure is:

• When X happens, Y happens.

This is a very common Japanese pattern for expressing regular outcomes, natural consequences, and general truths.