CONSTRUCTION OF Metro’s $6 billion, 23-mile Silver Line through Tysons Corner to Dulles International Airport and points west is about halfway through its first, 12-mile leg. The Metropolitan Washington Airports Authority, which is managing the project, insists that it will be finished on time, in mid-2013. But there are increasing signs that it may not be on time and possibly not on budget either.

According to the most recent projection by the general contractor, Dulles Transit Partners, the project is about six months behind schedule, up from a two-month delay forecast last spring. Both the contractor and the airports authority say that the project can, and will, be put back on schedule, but both acknowledge that doing so will cost money. And since more than two-thirds of the project’s $312 million contingency fund has been drained for changes and unforeseen events, money is tight.

It’s not unheard of for a venture on the scale of Dulles rail, one of the nation’s biggest infrastructure projects, to fall behind schedule or to suffer cost overruns. The trouble is that the airports authority has squandered much of whatever goodwill it once had from localities and Dulles Toll Road commuters, who together will bear most of the project’s costs — as well as any cost overruns. The authority did so with its political tone-deafness in pushing for a hugely expensive, utterly unnecessary underground Metro station at Dulles — rather than an aboveground one.

The authority lost that fight, but the project’s other stakeholders were left in an unforgiving mood. One effect may be to intensify scrutiny of the second phase, the financing of which remains a subject of negotiation. The airport authority’s board has insisted that the second phase be subject to a mandatory pro-union labor contract.

Until now, the authority has argued that a similar — but voluntary — deal in effect for the first leg of the venture has helped keep it on time and on budget. But with question marks hanging over both the schedule and the cost, it’s fair to question the value of requiring a labor agreement for the project’s second leg, which critics say would only drive up the costs.

Other unresolved issues remain. One is a frozen dispute between the airports authority and Metro over the scope of an expanded train maintenance facility at the West Falls Church station. The authority, under intensifying cost pressure, has pressed Metro to scale back the facility. Metro has refused.

The burden in delivering rail to Dulles lies with the project manager — the airports authority — whose management chops are already suspect. It must show that it is up to the job by completing the venture’s first leg without imposing major additional costs on its funding partners in Fairfax and Loudoun counties and by delivering a high-quality finished rail line to Metro with minimal delays.