I grew up in Takoma Park in the 60’s. They were talking about a rail line(going exactly where you said) then.
Metrorail does need a loop train to provide efficient intersuburban travel. The hub and spokes model on which all of DC's transportation systems were built has been inadequate for many years. How to finance a major expansion? Easy. How about a $10 a day tax on all parking spaces inside the beltway?
Employer-paid parking should also be counted as taxable income for the employee. Employer paid metro benefits should be treated the same way. Get rid of the subsidies to users. Or if that's too painful for the commuters to consider, make the subsidy universal, with employees who walk or bike or who save money with short metro or bus routes allowed to pocket the difference.