Learn Russian: Кто знает, когда был построен этот вокзал?

Построен is a short-form past passive participle. In Russian, when you create a passive sentence to say something 'was done' (like 'was built'), you use the short form. The long form построенный is mostly used as a regular adjective directly modifying a noun, like in the phrase 'the newly built station'.

Why is the word был required in this sentence?

A short-form participle like построен on its own describes a state in the present. To show that this action happened in the past ('was built'), you must add the past tense of the verb 'to be'. Since вокзал is masculine, we use был.

Why does the participle end in a consonant (построен) rather than -о or -а?

Short-form participles change their endings to agree with the subject's gender and number. The subject here is вокзал (train station), which is a masculine noun. Therefore, the participle takes the zero ending of the masculine form, построен. If the subject were neuter, like здание (building), it would be построено.

The English translation has 'when this station was built', but the Russian word order is 'when was built this station'. Is that mandatory?