موظف المحطة يعيد الحجز على القطار البديل.

It normally means the station employee.

That is because موظف المحطة is an iḍāfa construction, often called a possessive or noun-link construction:

• موظف = employee
• المحطة = the station

When the second noun is definite, the whole phrase becomes definite. So موظف المحطة is understood as the station employee.

Because in an iḍāfa construction, the first noun usually does not take الـ.

So Arabic says:

• موظف المحطة = the station employee

not:

• الموظف المحطة

The definiteness comes from the second noun, المحطة.
Also, the first noun in iḍāfa does not take tanwīn either.

المحطة is the second part of the iḍāfa, so it is in the genitive case.

In fully vowelled Arabic, it would be:

• مُوَظَّفُ المَحَطَّةِ

Notice the -i sound at the end of المحطةِ. In normal unvowelled writing, that case ending is usually not shown.

يعيد is the imperfect verb meaning he redoes, he repeats, or in this context he rebooks.

It comes from the verb أعاد.

So:

• أعاد = he redid / repeated
• يعيد = he redoes / is redoing

This is different from يعود, which means he returns / comes back.
That is a very common confusion for learners.

Because Arabic allows more than one normal word order.

This sentence uses subject + verb + object:

• موظف المحطة يعيد الحجز...

But Arabic could also say:

• يعيد موظف المحطة الحجز...

Both are grammatical. Starting with موظف المحطة gives a bit more prominence to the employee.

الحجز is definite here, so it refers to the reservation / the booking as something known in the situation.

Arabic often uses الـ with action nouns or abstract nouns when the speaker means a specific instance or a known process.

So يعيد الحجز means something like:

• he redoes the booking
• he rebooks the reservation

In travel language, الحجز على القطار is a natural way to say booking or reserving on a train.

Here على is part of the usual expression for making a reservation connected to that train. It is not mainly about physical position.

So:

• الحجز على القطار = booking on/for the train

If you said في القطار, that would more strongly suggest being physically inside the train.

Because adjectives in Arabic usually come after the noun they describe.

So:

• القطار البديل = the replacement train / the alternative train

This is the normal noun + adjective order in Arabic.

Because adjectives in Arabic agree with the noun they describe in definiteness.

Since القطار is definite, the adjective must also be definite:

• القطار البديل

If the noun were indefinite, the adjective would also be indefinite:

• قطار بديل

So Arabic matches them.

A fully vowelled version would be:

• مُوَظَّفُ المَحَطَّةِ يُعِيدُ الحَجْزَ عَلَى القِطَارِ البَدِيلِ

This shows the case endings more clearly:

• مُوَظَّفُ = nominative
• المَحَطَّةِ = genitive, because of iḍāfa
• يُعِيدُ = imperfect indicative
• الحَجْزَ = accusative, as the direct object
• القِطَارِ = genitive after على
• البَدِيلِ = adjective agreeing with القِطَارِ in case and definiteness