أعلنت المحطة عن قطار بديل، فانتظرت على رصيف القطار.

Because the subject is المحطة (the station), and محطة is a feminine singular noun in Arabic.

In the past tense, a verb with a she/it feminine singular subject takes this ending, so:

• أعلنَ = he announced
• أعلنتْ = she/it announced

So أعلنت المحطة means the station announced.

That is a very common word order in Modern Standard Arabic.

Arabic often uses:

• verb + subject + rest of sentence

So:

• أعلنت المحطة... = The station announced...

You could also say:

• المحطة أعلنت...

But the verb-first version is very normal and often sounds more neutral or narrative.

Here عن introduces the topic/content of the announcement.

So أعلنت عن... means something like:

• announced about...
• more naturally in English: announced... / announced that there was...

Arabic often uses أعلن عن where English would simply use announce with no preposition.

So:

• أعلنت المحطة عن قطار بديل
  means the station made an announcement concerning a replacement train.

Because Arabic adjectives normally come after the noun they describe.

So:

• قطار = train
• بديل = replacement / alternative

Together:

• قطار بديل = a replacement train

This is the normal noun + adjective order in Arabic.

Because it has to agree with قطار.

Arabic adjectives agree with the noun in:

• gender
• number
• definiteness
• case

Since قطار is:

• masculine
• singular
• indefinite

the adjective must match it:

• بديل

If the noun were feminine, the adjective would also be feminine.

Because the meaning is a replacement train, not the replacement train.

So Arabic uses the indefinite form:

• قطار بديل = a replacement train

If it were definite, it would be:

• القطار البديل = the replacement train

So the sentence is presenting it as something newly mentioned, not as a specific already-established train.

It means so, then, or and so.

It connects the second action to the first one and shows sequence or result:

• the station announced a replacement train,
• so I waited...

This فـ often suggests a fairly direct follow-up, more immediate than ثم in many contexts.

Because Arabic verbs already contain subject information.

In the past tense:

• انتظرتُ = I waited

So Arabic does not need a separate pronoun unless you want emphasis.

That means:

• فانتظرت already tells you I waited
• فأنا انتظرت would be more emphatic, not the neutral default

Yes, it can be.

Without short vowels, انتظرت could represent:

• انتظرتُ = I waited
• انتظرتَ = you (masculine singular) waited
• انتظرتِ = you (feminine singular) waited

In ordinary writing, Arabic usually leaves out those short vowels, so readers rely on:

• context
• surrounding grammar
• the known meaning of the sentence

Here, from the meaning, it is understood as I waited.

There are two useful things here.

First, على is the preposition used for being or waiting on a platform, so this matches English fairly closely.

Second, رصيف القطار is an iḍāfa (construct phrase), literally:

• the platform of the train

In natural English, that is often:

• the train platform
• the platform for the train

So على رصيف القطار means on the train platform / on the platform.

Because this is an iḍāfa structure.

In an iḍāfa:

• the first noun usually does not take الـ
• the second noun determines whether the whole phrase is definite

So:

• رصيف القطار = the train platform

Even though رصيف itself has no الـ, the whole phrase is definite because القطار is definite.

This is a very common Arabic pattern.

Not necessarily. This works like English.

In both English and Arabic, a place or institution can stand for the people or system connected with it.

So:

• المحطة أعلنت
  can mean the station staff, the station system, or the public announcement system made the announcement.

It is a natural kind of shorthand.

A fully vocalized connected reading could be:

أَعْلَنَتِ الْمَحَطَّةُ عَنْ قِطَارٍ بَدِيلٍ، فَانْتَظَرْتُ عَلَى رَصِيفِ الْقِطَارِ.

The important grammar points are:

• المحطةُ is nominative because it is the subject
• قطارٍ is genitive because it comes after عن
• بديلٍ matches قطارٍ, so it is also genitive
• رصيفِ is genitive because it comes after على
• القطارِ is genitive because it is the second noun in an iḍāfa

Also, in connected pronunciation, you may see أعلنتِ المحطةُ with -تِ before الـ. That is just a pronunciation/helping-vowel issue in continuous speech, not a different verb form.