But LaHood said the station, which first opened in 1907, "has struggled to keep pace with modern demands.
"With over 100,000 daily rail and transit trips, the station is already operating beyond its current capacity," he said. "And with projections showing that Amtrak and commuter rail ridership at the station could more than double in the next fifteen years alone, it's only set to get busier."
Amtrak said when it unveiled the master plan that the renovations to Union Station will make it easier for the station to accommodate high-speed trains in the future. The agency has called for increasing the speed of its trains to more than 200 miles per hour by 2030.
LaHood made a similar point on Monday.
"The expanded rail and passenger capacity in this station is essential to bringing high-speed rail to Washington--and to accommodating future growth on the Northeast Corridor," he wrote.
LaHood added that the proposed new train shed would "welcome passengers to the nation’s capital, bring natural light to station spaces, and better organize connections to Amtrak, commuter rail, transit and other transportation services.
"New passenger concourses and a series of new street entrances will be seamlessly integrated with the existing station, allowing passengers and visitors to easily access the entire station complex and adjoining neighborhoods while enjoying improved amenities and expanded retail opportunities," he said of the proposals.
LaHood said the Amtrak master plan called for the improvements to Union Station to take place over a 15-year period.