Social Security’s retirement system is like a speeding train full of happy passengers. It’s a comfy ride for the current leg of the journey. But the passengers may be clueless — or maybe don’t care — that a bridge is out just over the next rise.

Absent changes to the system, Social Security is headed into a chasm for the next generation. Baby boomers who’ve paid into the system for decades have begun drawing money out in big numbers, but there will be too few working Americans to keep the retirement fund in the black beyond 20 more years. The math just doesn’t work.

That’s the not-so-happy message as Social Security celebrates its 80 anniversary this month.

The good news is there is enough time to avert major pain if Washington takes steps early enough. It would take the kind of bipartisan bargain the Obama White House and GOP leaders in Congress could only yak and fantasize about.

The time is now — during the 2016 presidential campaign — to lay the groundwork for that politically dicey bargain.

Some in the Republican presidential field would bring common-sense reforms to the table. Former Florida Gov. Jeb Bush and New Jersey Gov. Chris Christie, for example, say they’d gradually boost the retirement age for younger people. Christie gets specific, saying full retirement eligibility should be delayed two years. He’s specific, too, on whether wealthy people should draw from the system. Christie says no, multimillionaires with retirement income over $200,000 don’t need monthly government checks to sustain them.

It’s hard to see how Social Security solves its math problems without these kinds of bold strokes. Bill Gates, Warren Buffet and Ross Perot don’t need government to keep them out of poverty, which was the original intent of the Social Security system.

Meanwhile, the presidential field contains many defenders of the status quo on Social Security, both Democrat and Republican. On the GOP side, former Arkansas Gov. Mike Huckabee would build a wall around Social Security as part of sweeping tax reform. That stance is not far from the recent statement from Democratic frontrunner Hillary Clinton: “We don’t mess with it.”

Even less realistic was a position outlined by 71 congressional Democrats in a letter to President Barack Obama last month; they asked his support to “expand Social Security benefits for millions of Americans.” The signees were joined by Sen. Bernie Sanders of Vermont, who’s challenging Clinton for the Democratic presidential nomination.

Nowhere did the letter say how proponents would fund more generous benefits. It’s delusional and irresponsible to ignore the math like that.

The facts are these: In 1960, five workers supported every Social Security recipient. Today, it’s fewer than three. In 20 years, it’ll be about two.

The financial stress will be enormous on people in the next generation, and it’s up to today’s leaders to make sure they don’t get crushed.