The Outer Beltway did have land in reservation up to the 1970's. But thanks to elected officials like the late Idamae Garrott, we don't any longer.
As I understand it, the reservation land was all east of I-270 because there was uncertainty on Virginia's part on where to put the river crossing, documented quite well in the 1969 Northern Virginia Major Thoroughfare Plan.
At more than a few meetings that led up to the never-completed 1998 InterCounty Connector Draft Environmental Impact Statement, the late state Sen. Idamae Garrott (D-19) proudly held forth about her efforts (back then, as a member of the Montgomery County Council) how she and her anti-highway and anti-mobility colleagues on that council would journey to meetings of the Loudoun and Fairfax County Boards of Supervisors and lobby them to remove any mention of new crossings of the Potomac River between the I-495 American Legion Bridge (known as the Cabin John Bridge back then) and White's Ferry. The implicit sales pitch she made then was that those same tactics would be successful in stopping the InterCounty Connector from ever being built (beyond the short I-370 segment, which she also opposed). She went to her grave in 1999 thinking she had won the battle of the ICC.
But Brian's right. There just isn't any good place to place a new crossing where it'd do the most good. Between suburban residential development on both sides of the river (especially in North Potomac) and the parks along/adjacent to the river, there is no way anymore to directly connect the ICC/I-370 with either 28 or Fairfax County Pkwy. And even if it were supported, it would be such a massively expensive endeavor that the real-estate costs alone makes it cost-prohibitive.
The subject has not been studied (on the Maryland side) since the early 1970's, when Garrott and her allies got away with removing the Rockville Facility and new crossings of the Potomac River from the Master Plan of Highways. Much of the land is not developed, and claims that it will "harm" the Montgomery County Agricultural Preserve are just that - claims. I-270 (completed and opened to traffic in the late 1950's as the U.S. 240 Washington National Pike freeway) has crossed the Agricultural Preserve since it was created by the Montgomery County Council with little public debate in the 1970's).
If you include the Great Falls area downstream and facilities on the Virginia side upstream, the physical reality prevents any new crossing between the Beltway and about Leesburg. Both the political and fiscal realities are such that the best we can hope for is improvements to the existing American Legion and Point of Rocks Bridge crossings.
Great Falls is a park on both sides (as is the entire C&O Canal on the Maryland side from Cumberland to Georgetown). But the American Legion Bridge has not adversely affected the C&O Canal and its towpath.
I agree that the land use does not support Metro. But there's enough latent demand plus intervening smaller nodes to where an LRT extension of the planned Purple Line from Bethesda to Tyson's becomes feasible.
I disagree, for the following reasons.
(1) You think there's well-funded NIMBYist opposition to new river crossings? I suggest you consider communities west of Md. 355 (Wisconsin Avenue) and all along Md. 190 (River Road). They are going to oppose any rail transit project that runs near their homes.
(2) The demand for travel is not from Bethesda to Tysons Corner. The Metrobus Route 14 service that was 100% funded by Maryland (thanks to then-Gov. Parris Glendening) had no patronage at all, and was discontinued shortly after he left office for that reason. People wanting to get from Maryland to jobs along the Va. 267 corridor are not coming from downtown Bethesda.
(3) A lot of the heavy traffic on the Beltway between Va. 267 and I-270Y in the afternoons is due to Dulles Airport, which is an
afternoon peaking airport because of all of the international traffic first arriving from Europe and then departing a little later in the evenings.
(4) If Montgomery County is going to have any chance of attracting more and new private sector jobs, it needs to upgrade its ground access to Dulles Airport. Managers of private-sector firms that make location decisions are just not that interested in being near rail transit stops, no matter how fervently planners and the elected officials that oversee them would like to believe that. In spite of Montgomery County's claim of being committed to Life Sciences, the American Type Tissue Collection moved from Rockville to Prince William County just outside of Manassas (moving them significantly closer to Dulles), and the Howard Hughes Medical Institute (long headquartered in the Chevy Chase area of Montgomery County) did not even consider Montgomery County when they established the Janelia Farm Research Campus in Ashburn, Loudoun County, overlooking the Potomac River (and Montgomery County's Agricultural Preserve) and a short, easy drive to Dulles).
(5) Finally, you are making an assumption that has been going on in the metropolitan Washington area since the 1960's, that rail transit can replace limited access freeway-class roads. I believe that is wishful thinking. The biggest beneficiary of such have been the counties of Northern Virginia, especially Fairfax and Loudoun Counties, but also Arlington County (home to National Airport and the Pentagon) and Prince William County.