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Comparable facilities recently completed or currently under construction in Spain, France, the UK, and elsewhere include parking for up to 4,000 cars.
As the West Loop continues to densify, and as the land around Union Station becomes more valuable, it is unrealistic to expect that existing surface parking and parking garages -- created when the West Loop was less desirable -- will continue to be available for parking for Union Station. As has been happening throughout the city, these lots will most likely be dedicated to more profitable uses.
It is therefore imperative that Union Station have its own parking to deal with an annual passenger load roughly equal to New York's JFK airport.