حالما يصل القطار البديل، أذهب إلى رصيف القطار.

حالما means as soon as or once.

In this sentence, it introduces a time clause:

• حالما يصل القطار البديل = as soon as the replacement train arrives

It is a very common way to connect two actions when the second happens immediately after the first.

In Arabic, the present tense often covers both present and future, especially when the future meaning is already clear from context.

Here, حالما makes the timing clear, so يصل is understood as a future event:

• حالما يصل = as soon as it arrives

Arabic does not always need a separate future marker like سـ or سوف when a time expression already shows that the event is in the future.

For the same reason: the sentence already clearly refers to a future situation.

So:

• أذهب can mean I go or I will go, depending on context
• after a time clause like حالما يصل..., it is naturally understood as future

You could say سأذهب if you wanted to make the future meaning more explicit, but أذهب is perfectly normal here.

Arabic verbs often include the subject inside the verb itself.

In أذهب:

• أـ marks I
• ذهب is the basic verb idea of going

So أذهب by itself already means I go / I will go.

That is why Arabic does not need a separate subject pronoun here. You could add أنا for emphasis, but it is not necessary.

البديل means the alternative, the substitute, or the replacement.

So:

• القطار البديل = the replacement train / the alternative train

This is an adjective describing القطار.

In Arabic, adjectives normally come after the noun they describe.

So:

• القطار = the train
• البديل = replacement / alternative

Together:

• القطار البديل = the replacement train

This is the normal Arabic order: noun + adjective.

Because Arabic adjectives usually agree with the noun in definiteness.

Since the noun is definite:

• القطار = the train

the adjective must also be definite:

• البديل = the replacement

So Arabic says:

• القطار البديل

not:

• القطار بديل

This is a standard rule for adjective agreement.

Yes, when ذهب means to go somewhere, it is usually followed by إلى:

• ذهب إلى... = went to...
• أذهب إلى... = I go to... / I will go to...

So:

• أذهب إلى رصيف القطار = I go to the train platform

This is the normal preposition used with ذهب for movement toward a place.

This is an idafa construction, often called a possessive or noun-link construction.

• رصيف القطار literally means platform of the train
• in natural English, that becomes the train platform

In an idafa:

1. the first noun usually does not take الـ
2. the second noun can be definite

So:

• رصيف القطار = the train platform

Even though رصيف has no الـ, the whole phrase can still be definite because القطار is definite.

Arabic often repeats the noun when it helps specify meaning clearly.

• الرصيف by itself could just mean the platform
• رصيف القطار specifically means the train platform

So the repetition is not strange in Arabic. It makes the destination more precise.

Depending on context, Arabic could sometimes shorten it, but the full phrase is clear and natural.

A simple breakdown is:

• حالما — conjunction meaning as soon as
• يصل — verb, arrives
• القطار — subject of يصل
• البديل — adjective describing القطار
• أذهب — main verb, I go / I will go
• إلى — preposition, to
• رصيف — noun after a preposition
• القطار — second noun in an idafa, of the train

So the structure is:

• time clause: as soon as the replacement train arrives
• main clause: I go to the train platform

In fully vowelled, careful MSA, it would be:

• حالما يصلُ القطارُ البديلُ، أذهبُ إلى رصيفِ القطارِ.

The endings are:

• يصلُ — indicative present
• القطارُ — nominative, because it is the subject
• البديلُ — nominative, matching the noun it describes
• أذهبُ — indicative present
• رصيفِ — genitive after إلى
• القطارِ — genitive as the second part of the idafa

In everyday unvowelled writing, these endings are usually not written.

It can technically depend on context, because the Arabic present tense is flexible.

But in this sentence, most learners will understand it as a future action because of the time clause:

• As soon as the replacement train arrives, I will go to the platform

Without more context, it sounds like a planned action in the future rather than a general habit.