A couple of things come into play. Today we put all our guests on the train and we run down toward Seattle and we disembark everybody at the same time. Imagine 300 people showing up at once with a handful of customs agents to greet those 300 people. It becomes quite a long process.
Doing it at our station allows us to manage that relationship with the guests as people arrive at the station, make them comfortable and run them through the queues at our leisure, as opposed to all at once in a herd.
The second part of pre-clearance on the good side is a lot of freight traffic runs between Seattle and Vancouver as well, so if the goods also have a pre-clearance and that can run smoothly through the border that won't hold us up as we travel.