If you want to identify a tail, look at their shoes: they are hard to change. Move frequently between crowded and empty places: this forces them to keep closing for fear of losing you, drawing back, then closing again. This makes them conspicuous. But don't jump on or off trains just before the doors close--that's for the movies; and anyway, a good surveillance team will already have someone on the train, as well as on the platform. Remember they're not trying to catch or chase you, just to "house" you--to see where you live, where you're going, or whom you're meeting.