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“We had a huge community process between 2003-06,” Ms. Hynes said. “Residents were deeply engaged in the options that were available, and we got support from a large and significant majority of people.”

According to the department, the projected increase of housing values appears unlikely to cause widespread displacement or dramatically transform neighborhood character. In Arlington, officials are adamant that the affordable housing stock in the affected area — currently about 25 percent — will stay the same.

“We want to end up with the same number of affordable units,” Ms. Hynes said.

In other cities, the development of streetcar lines often is spurred by a combination of federal, state and local funding, with the federal money often coming from the discretionary New Starts and Small Starts program. Arlington and Fairfax counties are using this kind of mix for their streetcar projects. The District is relying on local investment, according to Ms. Ward but is not ruling out the possibility of federal funding in the future.

The mix of funding is part of the concern, detractors say, because blending funds often masks the real costs of such projects.

According to George Mason’s Ms. Norcross, some of the building costs can be externalized, meaning passed on to state and federal taxpayers, many of whom won’t be enjoying the services.

The Arlington County Board estimates that the Columbia Pike streetcar will cost between $214 million and $261 million to build and between $19.5 million and $25 million to operate.

The jury is still out on whether the projects will be worth the expense. Proponents point to increased development; opponents note declining ridership in existing networks and rising costs.

But Ms. Hynes pointed out that rising costs are a fact of life.

“Of course costs have increased from what they were,” she said. “It’s just the reality. But lots of people thought Metro was too expensive. You have to make those investments for the long term.”