The Taggart Bridge, in Ayn Rand's novel Atlas Shrugged, was a railroad bridge spanning the Mississippi River that became, in the last years of the strike called by John Galt, the last remaining railroad connection between the eastern and western halves of the United States. When the detonation of Project X destroyed this bridge, the populace panicked and the country descended into anarchy.
The younger Taggart almost was not allowed to build it. Barge operators local to the town of "Bedford, Illinois" (actually, Rock Island is the more likely location) sued Taggart for "tortious interference with livelihood," in that they would lose their business to the railroad. They sought an injunction directing Taggart to stop building his bridge and use barges to ferry his trains across the river. Taggart fought the suit, though he lost at the district court level. At one point, his entire workforce deserted him. In response, he personally took tools in hand and tried to build his bridge single-handedly when he wasn't appearing in court.
Eventually, the Supreme Court of the United States reversed and remanded the case, 5-4. On the second trial, Taggart won and was allowed to continue building his bridge.
The Atlantic Southern Railroad also built a bridge over the Mississippi. But in the winter of 2019, flood-borne logs knocked the bridge out. The Taggart Bridge remained as the only functioning Mississippi River crossing in the entire country.
As the economy of the United States deteriorated rapidly, under the stress of so many talented individuals abruptly quitting and vanishing, the Taggart Bridge remained the symbol of all that was left of the American economy. While Dagny Taggart was staying at Galt's Gulch in June of 2019, John Galt warned her that, if she stayed in the Gulch permanently, he would inform her of the "eventual" collapse of this bridge. On her last day in the Gulch, Midas Mulligan pleaded with John Galt not to return to New York City in this last year, saying that the Taggart Bridge would collapse, and anarchy would surely result in that event. Dagny abruptly said that the bridge would not collapse on her watch, and that was her signal that she had decided to return to "civilization."
In the weeks that followed, politicians from Mr. Thompson on down repeately expressed their fears that "if anything happens to that bridge," the society over which they held the power would die, and their power would die with it. And with that power would go their own chances of personal survival.
Project X, the infamous weapon of mass destruction built by the State Science Institute, was located in Dunkertown, Iowa (renamed Harmony City) about a hundred miles northwest of the Bridge. Thus the Bridge was at the southeastern limit of the Xylophone's range.
On March 20, 2020, Robert Stadler, the (nominal) Director of the Institute, crossed the Taggart Bridge into Iowa in a vain attempt to seize Project X for himself and rule it as his private feudal domain. But when he arrived, he found a renegade militia, led by Cuffy Meigs, in control. The two men struggled physically over the controls, and in the process Meigs yanked one lever too many.
The Xylophone's sound rays struck out in all directions and flattened everything within the hundred-mile range. The Taggart Bridge was cut in half. With the northwestern half went the first six cars of a passenger train.
News of its collapse spread swiftly. Dagny Taggart witnessed the aftermath in New York City. Nearly every resident loaded his belongings onto the roof of his automobile and attempted to drive out of the city—for without reliable railroad service, no city could survive long, especially New York.
Shortly after that, power failed from one end of "Megalopolis USA" to the other.
The novel does not make clear exactly how John Galt and his fellow strikers restored the economy following the collapse. Dagny Taggart, in her efforts to rebuild Taggart Transcontinental, would certainly have rebuilt this bridge—and probably built it of Rearden Metal.
Spoilers end here.
The Taggart Bridge, in this novel, is a type of any particular part of the old order to which people will cling, when perhaps they should relinquish it and trust God, or in Ayn Rand's philosophical scheme, their own judgment, rather than the judgments and achievements of others. One cannot build a new order without destroying the old. The Bridge is, therefore, a crutch, and when it fails, the rest of the economy fails with it. Dagny Taggart recognizes that failure as her cue to join the strike, and the ordinary citizens recognize that the old governmental order, in which they had reposed so much trust, cannot help them.