CAMBRIDGE — America’s fiscal position is increasingly looking like a train hurtling along a twisting mountain track. No one seems willing to slow down for fear of falling behind schedule and losing their job. Even if the train doesn’t crash, it is worth asking: Why keep pushing it? Despite solid growth, the federal deficit is about 6 percent of GDP. Yet the option of slowing down does not appear to be in President Donald Trump’s playbook. The start of a public-debt crisis is usually pretty straightforward. The end is much harder to predict. A crisis can erupt when a country’s debt is historically high but has remained manageable thanks to low interest rates. Then, for whatever reason, rates begin to rise. It might be because of global forces, a change of government, or simply because memories of previous financial crises have faded. This is roughly where the United States finds itself today. Rising borrowing costs would be less of a problem if the government had locked in cheap long-term financing, but it didn’t. Most homeowners, by contrast, did. They are still enjoying lo