The crowning of Chicago firm Riverside Investment & Development as the leader of a massive Union Station redevelopment answered two key questions: Who won? And what will they build?



Amtrak President and CEO Wick Moorman, Mayor Rahm Emanuel and other officials announced Thursday the selection of Riverside and its $1 billion-plus project that is expected to include up to 2 million square feet of office space, 780 apartments and 350 hotel rooms.



The biggest unknown is whether the development team led by John O'Donnell's firm can pull off a 3.1 million-square-foot project, after previous plans by other developers stalled.



The last such proposal, in which the American Medical Association a decade ago planned to move into an 18-story office and hotel tower that would be built atop the train station, was felled by a recession.



Timing once again may be a key consideration, since Riverside's three-phase proposal is expected to take six years to complete, starting in mid-2018. It will launch after an already bullish run of residential, office and hotel construction over the past several years. The plan is further complicated by the need to build over the nation's third-busiest rail station and active rail lines along the building.



Despite those challenges and the building's fortresslike exterior, O'Donnell said he was optimistic about the location.



"It's something you want to walk by fast," O'Donnell said. "And that's the opportunity. What we want to have is outward-facing retail and a very comfortable pedestrian experience. From the developer's perspective, that's the first goal, to create a sense of place and make it very comfortable and inviting."



Working in Riverside's favor is O'Donnell's history — first as an executive at John Buck Co. and more recently at his own firm — of executing large, complicated projects over and along train tracks. And Riverside could benefit from big changes already underway in the area.



Union Station, which was envisioned by Daniel Burnham and designed by Graham Anderson Probst & White, opened in 1925. It is in an area long considered no man's land between Loop skyscrapers and the Kennedy Expressway. Yet its transformation is planned at a time when other developers are planning billions of dollars in real estate projects nearby.



Directly east, across the river, Blackstone Group acquired the 110-story Willis Tower for a Chicago-record $1.3 billion in 2015 and has announced plans to spend another $500 million upgrading the building. The plan includes building a 300,000-square-foot base on the tower for retail, restaurants and entertainment venues.



Immediately south of Union Station, New York developer 601W Cos plans an approximately $500 million redevelopment of its own, of the hulking old main post office. The New York developer bought the vacant post office last year for $130 million and is seeking office and retail tenants amid ongoing construction work.



A few blocks south of there, Related Midwest said it plans to spend at least $5 billion turning a vacant 62-acre parcel between the South Loop and Chinatown into residential, office and hotel towers, retail, a wide riverwalk and other public space over the next 20 years.



Just west of Union Station, a venture including Chicago firms White Oak Realty and CA Ventures is building a 20-story office building on speculation, or without tenants signed in advance, near Old St. Patrick's Church.



And farther across the Kennedy, the Fulton Market district is experiencing a flood of development that is transforming the former meatpacking and food distribution hub into an area for dining, shops, boutique hotels and offices for major corporations such as McDonald's and Google.



Union Station's first phase of development will include 110,000 square feet of new and reconfigured retail space, including a food hall. The station's eight-story headhouse, or main building, will be renovated to include 100,000 square feet of office space and a 350-room hotel in existing floors above the Great Hall that are vacant. The main building will be topped by a pair of new 12-story residential towers.



In the second phase, two office towers of at least 750,000 square feet each — and possibly as large as 1 million square feet each — will be built south of the main building atop retail and 800 parking spaces. That will replace a 1,600-space parking garage now on the site. Terraces and plazas will be created above the current bus transit center.



In the third and final phase, a 500,000-square-foot residential and retail tower is planned over rail lines at the southeast corner of Jackson Boulevard and Canal Street.



Other companies involved in the project include Convexity Properties, which is the real estate development arm of trader Don Wilson's DRW Holdings, and architect Goettsch Partners.



Convexity's developments have included a 50-story Streeterville building that includes apartments and the Loews Chicago Hotel, the Esquire Theater retail redevelopment, and Robey and Viceroy hotels.



Convexity and Riverside are contributing "substantial equity," O'Donnell said. He said the firms still need to arrange construction financing, and may bring in additional investors. No public money is expected to be used in the project, O'Donnell said.



"Our goal was to have the equity in place for the first phase when we made our proposal to Amtrak, so they could be assured that once we get the documentation and the government clearance is completed, we're going," O'Donnell said.



Riverside recently completed a 53-story office tower along the Chicago River, and also has announced plans to start building next year a 51-story riverside office building anchored by Bank of America. That development will be on the site where mall owner GGP's Wacker Drive headquarters is set to be demolished.



Amtrak, the city of Chicago, Metra and the Regional Transportation Authority last year chose four finalists for the project. The other development groups were led by Chicago firms Sterling Bay, John Buck and Golub & Co.



Amtrak said it plans to negotiate final terms of the development agreement with Riverside by the end of this year.



Riverside's project is expected to create about 7,500 construction jobs, plus 7,000 to 8,000 permanent jobs, Amtrak said.



The Chicago redevelopment is the first step in a nationwide effort by Amtrak to upgrade its stations, Moorman said. Improvements also are planned in cities including New York, Philadelphia, Washington and Baltimore.



"It's incumbent upon us at Amtrak to take our stations to the next level," Moorman said. He added, "We want to build a new West Loop gathering place."



The first step will be to change the main building's exterior and encourage more pedestrian traffic in the area. The layout of the station also will be changed to improve the flow of passengers, O'Donnell said.



Improved retail, open spaces, hotel guests and residents will help attract office tenants, allowing the developers to secure construction financing for the new office towers, O'Donnell said.



"This is really a transformative development," O'Donnell said. "This is probably one of the best physical locations in the city. It just needs to be dressed up, and I think it needs to have a number of amenities that don't exist right now. We can bring an abundance of those to this location."