Try reading the sentence like this to make sense of it: "There were porters who seemed to be engaged with mail bags and luggage vans". The writer is is trying to make a point about the type of porters there were and the kind of work they engaged in. Hope this helps!

There is more to this phrase than the explanation given by @Stallone. The phrase "Such porters as there were" implies that that the porter service was inadequate, and that even the few porters who were present were already busy with mail bags and luggage vans. This is a neat way of giving two pieces of information in one sentence: (1) There weren't enough porters (2) The (small number of) porters were busy. Here is another example of this phrase: "Such food as there was looked cold and unappetising." This tells you two things: (1) There wasn't much food (2) The food looked cold and unappetising. I hope that's clearer now. NB A word of advice: You should wait a while before clicking 'Best Answer'. The first answer you receive is not necessarily the right one!