Even if they don’t win the contract to build the second phase of the Dulles Rail extension, the five firms selected Wednesday to bid on the project could each be eligible for a $1.5 million check from the Metropolitan Washington Airports Authority.

MWAA officials said it is common practice to offer a stipend to firms that bid on pieces of “mega projects,” like the $5.6 billion rail line.

“It’s considered to be the standard in industry,” said Eric Carey, contracting officer for the Dulles Corridor Metrorail project. “We need to offer a stipend in order to get competitive bids. Otherwise, we won’t get the response we want or won’t get competitive pricing.”

However, some experts familiar with federal contracting were surprised to learn of such a practice. “That is extraordinary,” said Steve Schooner, a professor at George Washington University and former associate administrator for procurement law and legislation at the Office of Federal Procurement Policy in the Office of Management and Budget.

Around the world, companies prepare bids for all sorts of projects without that sort of incentive, he added. “It’s the cost of doing business,” Schooner said.

One of the five firms selected by MWAA to bid on Phase 2 is Bechtel Transit Partners, the contractor in charge of Phase 1 of the rail project. The other four are Capital Rail Constructors, which includes Clark Construction Group and Kiewit Infrastructure South Co.; Dulles APC Railbuilders, which includes Archer Western Contractors, PCL Civil Constructor Inc. and Corman Construction Inc.; Dulles Metrorail Connectors, which includes Skanska USA Civil Southeast, Granite Construction Company, G.A. & F.C. Wagman Inc., Trumbull Corporation and Facchina Construction Company Inc.; and Silver Line Constructors, which includes Fluor Enterprises, Inc., Tutor Perini Corporation, and Stacy and Witbeck Inc.

MWAA officials said they expect to award the contract next May on Phase 2, which would bring the planned Silver Line to Dulles International Airport and into Loudoun County. The first phase of the project, which runs from East Falls Church through Tysons Corner to Reston, is slated to be completed in August 2013.