Of course that should be accepted, it would actually be a more logical translation. Few people would ever wait for the arrival of any random train, most would be waiting for a specific one, especially if it is an international train.
Well, I personally am still a bit hesitant to accept it. That doesn't stop any other contributor to accept it though. I still feel like I would expect "toho mezinárodního vlaku" for "the international train".
I would say that there's not enough context here to make it clear whether 'a' or 'the' is better, both are possible. To be honest, I think one would be more likely to say 'I'm waiting for the (time) train' or 'I'm waiting for the train to (place)' than this.