So much has been written about Abraham Lincoln that it’s rare when a historian discovers an episode in his life that, if fully developed and interpreted, yields important new insights. Ted Widmer has done just that in his superb new book, “Lincoln on the Verge: Thirteen Days to Washington.” It’s ostensibly about the train trip the president-elect took from Springfield, Ill., to the nation’s capital; it’s in fact about how Lincoln and his fellow Americans came to know and trust one another, an experience that profoundly shaped his presidency.

In February 1861, Southerners feared that Lincoln’s election meant the end of slavery and their way of life; talk of secession and civil war was rampant. Northerners rejoiced at the election’s outcome, while fearing that it could lead to the country’s dissolution; they looked to Lincoln for reassurance.

Widmer, a historian at the Macaulay Honors College of the City University of New York, spent 10 years applying his impressive talents as a researcher and storyteller to explain how our 16th president used a 13-day train trip to introduce himself to his fellow citizens and to prepare himself for the greatest crisis in the nation’s brief history.

The issue of slavery, most believed, would determine future events. Though his views were well known, Lincoln adopted a “strategy of silence” during the campaign and now through the four-month interregnum. But the Republican platform was unequivocal: Slavery must not be extended beyond the states where it already existed. “That was the rock that now loomed before the ship of state,” Widmer writes. As their next president, Lincoln believed he must calm anxious citizens — but the Constitution left him no means to do so.

Lincoln chose to use his train trip as the vehicle for connecting with those who elected him. Widmer demonstrates a deft ability to relate Lincoln’s circumstances to those of others in the nation’s past: He quotes George Washington as feeling like “a culprit who is going to the place of his own execution” on the eve of his inauguration in 1789. “But his long trip from Mount Vernon to New York had helped to make his presidency real to the people. Now, in order to save the country, Lincoln needed to summon all of his strength for an even longer journey . . . and he needed to get there quickly while there was a country left to save.”

In plotting his route, however, speed was not his priority; political sagacity was. He insisted on visiting the capitals of the five “essential” states that had elected him — Indiana, Ohio, Pennsylvania, New York and New Jersey; if hostilities broke out, he would need them to provide the manpower to deal with it.

On Feb. 11, an emotional Lincoln bid “an affectionate farewell” to 2,000 friends before the “Presidential Special” headed out. Hundreds and then thousands lined the tracks, trying to catch a glimpse of the man who embodied their hopes. These were “Lincoln’s kind of people,” in Widmer’s telling, “and he reached out toward them every few miles, waving, or bowing, or shaking hands, or saying a few quick words sometimes from a train that slowed but did not stop.”

As Lincoln had plotted his route strategically, so too does Widmer with his writing; his creative structure and new research offer compelling diversions about some of the people and history the president-elect encountered. Included are past and future presidents, as well as the slaughterhouses of Cincinnati and the nauseating corruption of Albany. Every place had someone or something distinctive, and Widmer invariably finds it.

The trip proved exhausting and at times frightening — with at least two assassination attempts. Lincoln was often in pain, Widmer says, “especially his hands, after the nightly ordeal of shaking hands with thousands of local well-wishers.” Nonetheless, “the trip was making a difference. Even when the president-elect said nothing, the response was overwhelming, as at Ashtabula, when he could barely speak and the crowd burst into a ‘state of din-bewildered enthusiasm,’ screaming simply because he was there.”

Lincoln’s confidence grew as the Special entered New Jersey, where he told state legislators that, while he favored reconciliation, “I fear we will have to put the foot down firmly,” which he suddenly did, literally and dramatically — to the cheers of the astonished lawmakers. “This was the clear statement everyone was waiting for,” Widmer writes. “. . . He had found his footing, in every sense.”

In Philadelphia’s Independence Hall, Lincoln “talked humbly about the way he understood the Declaration [of Independence], and the hope it inspired . . . the equal rights that inhere in all people.” The key word in the declaration, he added later, was “all” — “All men are created equal.” “It permits no equivocation,” Widmer adds, before asserting, “Lincoln had reset America’s moral compass.”

Lincoln ended his remarks with a “stunning” admission: He would be “one of the happiest men in the world” if the country could be saved with its great idea intact. He would “rather be assassinated on this spot than to surrender it.”

The evening before, detective Allan Pinkerton had warned Lincoln that an armed mob would be waiting for him in Baltimore and that he must deviate from his course to evade the “death trap.” Given a choice between continuing his journey with his dignity and his purpose intact, or avoiding assassination by stealth and probably inviting ridicule, he quickly chose the latter; the Special completed its journey, safely, to Washington.

“No one knew yet what a Lincoln presidency would mean, but the fact that he had survived the ordeal meant that his presidency would actually begin,” Widmer writes. “His odyssey was complete.” Days later, Lincoln arrived at the Capitol for his inauguration to speak of “the better angels of our nature.”

Widmer has written a revelatory work about an important but underappreciated episode that placed Lincoln “on the verge” of developing the confidence and courage to become America’s greatest president. His book could also be on the verge — of becoming a Lincoln classic.

Lincoln on the Verge

Thirteen Days to Washington

By Ted Widmer

Simon & Schuster. 606 pp. $35