I refer to the break-down of Social Security and Medicare, discussed here. (Thanks, Instapundit.)
One scenario is less a train wreck than a gradual slowing down of the train to the point that its momentum is nationally sustainable. (This could well cause thousands of train wrecks among individual citizens who decided to depend on the present system for their futures.) What happens will depend in large part on generations younger than the Boomers assuming power and exercising it more responsibly than my generation has done so.
For some reason, this reminds me of two grown-up women I know, one recently divorced and the other recently a widow. In each case, they received a fund, either as a divorce settlement or a legacy. Neither had a career to sustain her economically. On the other hand, each had her respective fund. If the divorcee and the widow kept their life styles in check and prudently managed their wealth, the funds would probably see them through. But each rewarded (consoled?) themselves within a year or so after the event with very, very expensive overseas vacations. They otherwise do not appear to be readjusting their consumption levels. This doesn't seem to auger well to me. It seems like a form of denial.