The second phase of the Metrorail project at Dulles International Airport in Virginia, US, will reportedly cost $3.83bn, an increase over its earlier projected cost of $2.5bn.
The second phase, set to finish by the end of 2016, will include construction of a two-mile tunnel, one underground station at Dulles Airport and two in Loudoun County.
The Metropolitan Washington Airports Authority will take a final decision on alternative alignments and stations, including an aerial station, for Dulles Airport on 7 October.
The alternative alignments and stations could shave $640m off the project's costs, according to consultants.
Construction of the first 11.5 miles of the rail line is underway and is expected to be complete by December 2013.
The first phase of the project will cost $2.7bn to complete, according to loudountimes.com.