There are several problems with this sentence. The most important is the sequence of tenses. The problem with the train occurred in the past. Therefore you must say there was rather than there is. Secondly, it is unnatural to say there was a short delay of the train. We would naturally say my train was slightly late. Next, the use of because is inappropriate here. You should break it into two sentences. I am sorry to be late. My train was slightly late. There is no need to speculate on the reason that the train was late. Ordinarily passengers do not know the reason anyway.