The subway, the train, the T, the underground, the metro, the tube-whatever you call it, it's how we get around. Boston's happens to be the first, and when one has the world's most ancient subway system, it's easy to dismiss it as old news. But the MBTA has a big birthday this year, and it deserves its rightful centennial celebration. For the week, we played "I Spy." This is what we saw.