I purchased 2 tickests in advance online, but then we met a friend in the City for lunch who wanted to join us. The tour guide allowed a "walk-up" and she was able to pay and take the tour. We also met nice people from California in our group and from the town next door on Long Island. Wow! Our guide herded us around the busy station with ease, told stories, answered questions, and made the entire experience a delight. We loved it and would recommend it to locals.