RESTON, VA - JULY 28: Commuters ride the silver line metro during the first business day of operation for the line on Monday, July 28, 2014 in Reston, VA. (Photo by Amanda Voisard/For the Washington Post) (Amanda Voisard/For the Washington Post)

THINK METRO trains are crowded at rush hour now? Just imagine the not-implausible future if the transit system’s state and local benefactors fall behind on their funding commitments for capital upgrades, including Metro’s scheduled acquisition of new rail cars.

Metro’s just-opened Silver Line and the region’s growth projections make it clear that a crush of new riders is on the way. The average weekday ridership is in the range of 700,000; that is forecast to rise by at least 100,000 by 2020. According to Metro’s published documents, just one of the network’s six existing lines (the Orange Line) is currently rated as “crowded,” meaning 100 to 120 people crammed in a rail car during rush hour. Now fast-forward to 2021 and assume a delay in funding to purchase new rail cars. The Red, Green and Blue lines would all be crowded, and the Orange Line would descend into the infernal precinct of “extremely crowded,” meaning more than 120 people per car at rush hour.

In other words, just seven years from now without adequate investment in Metrorail, every day will feature crowds on Metro rivaling a presidential inauguration. Given the area’s projected rates of population and job growth beyond 2020, the picture gets only worse in later years.

Currently, just a third of the 140 trains running on Metrorail’s tracks are equipped with the maximum eight cars at rush hour; the rest run with six cars. Metro’s goal is that every train in its fleet will run at peak hours with eight cars by 2021. To get there will require nailing down funding commitments to buy 220 cars.

That purchase must be finalized by June 30; if it’s not, the contract with the manufacturer, Kawasaki Rail Car , would probably dissolve. That would mean going back to square one — new bids, new contracts, new engineering — and a delay of at least five years in Metro’s expansion plans, to say nothing of spiraling costs.

The 220 new rail cars, with the infrastructure to support them, will cost nearly $1.5 billion over six years, on top of existing funding commitments for modernizing the system from Metro’s main local benefactors: the District, Virginia and Maryland. A particular question mark is Maryland, which, despite new gas tax revenue, looks to have over-promised for the above-ground Purple Line in Montgomery and Prince George’s counties, the Red Line in Baltimore and an array of highway projects.

Officials in all three jurisdictions have paid lip service to the importance of expanding Metro, but obtaining final approvals for expanding the system may be a struggle. Mindful that the region’s growth could be crippled by below-par transit service, business groups have launched a lobbying and public relations campaign to rally support for increased funding. Along with the promised benefits, they might also offer gentle reminders of what a failure to provide needed funding will mean for Metrorail’s future — if anyone can see it through the throngs of riders.