Learn Dutch: Er komt straks een tram aan. - There is a tram arriving soon.

Because een tram is an indefinite noun (meaning a tram, not the specific tram). When introducing a new, unspecified subject to say that it exists or is happening, Dutch uses er (there) as a dummy subject to start the sentence, pushing the real subject later into the phrase.

Where does the word aan at the end come from?

It is part of the separable verb aankomen (to arrive). In standard Dutch main clauses, the conjugated part of the verb (komt) takes the second position, and the prefix (aan) gets kicked to the very end of the sentence.

Could I change the order and say Er komt een tram straks aan?

That sounds unnatural. In sentences that use er as a dummy subject, time words like straks (soon) almost always come before the actual subject (een tram). The standard pattern is: er + verb + time + subject.

Does straks mean straight away?

It is a common false friend! Even though it sounds a bit like straight, straks actually means soon or later (usually later the same day). It implies a short delay, not immediately.