Brother vs. Sister

John has two siblings, a brother and a sister, whom he both loves very much. Every Saturday, he takes the subway to visit one of them. To visit his brother, John takes a train on the downtown side of the platform. To visit his sister, he takes a train on the uptown side of the platform. He likes both his siblings equally well, so he simply takes whichever train first comes along. He arrives at the platform at a random time each Saturday. Both trains arrive at the station equally often – every 10 minutes. However, John realized that he has been spending more time with his brother than his sister (9 times out of 10). Why do the odds favor John's brother so much?