"It was an unforgettable 'Eureka!' moment," said Jonnes, Baltimore author of the new book Conquering Gotham - A Gilded Age Epic: The Construction of Penn Station and Its Tunnels. "I was looking through files that no one had seen in 100 years."
But the records described more than the pharaohlike achievement of Philadelphia's PRR president, Alexander Cassatt, more than the roles of titans including John D. Rockefeller, Andrew Carnegie, J.P. Morgan, William Randolph Hearst and Theodore Roosevelt.
They contained a surprise: The engineers were deeply concerned about the "up-and-down movement of the [Hudson River] tunnels 40 feet below the river bed," Jonnes said.
"They didn't realize how soupy the silt was, or how the tunnels would behave. They didn't know whether they would keep settling in the silt and crack."
In 1907, PRR engineer Samuel Rea, whose junior engineer son was lost to a deadly disease contracted in the tunnels, was stunned to read an editorial in Scientific American titled "Tunnel Tubes in Soft Material." It posed questions about the "bending stresses developed . . . far in excess of the resisting power of the tubes . . . fracture must ensue."
The Inquirer also wrote an editorial: "Here is a question in which the public is vitally interested not only through its ownership of the securities which built the tubes, but because it is to become a vast artery of travel. The thought of a breaking down of the tunnel is too horrible for contemplation."
Some engineers proposed a system of anchoring the tunnels in the bedrock, Jonnes said. It was never done.
In the end, thousands of 700-ton trains passed through before they were reassured that tunnels were not sinking.
Today, Philadelphia area residents still make the daily commute to jobs in New York. "They are going through these same tunnels," Jonnes said.
The tunnels were the dream of Cassatt, whose PRR brought him to the Hudson River - but could not take him across to Gotham. For that, he had to board ferries.
Cassatt had been the first to extend the PRR lines to Jersey City in 1871 and he helped develop the system of ferries. Now the ferry trip was galling to him.
"No one in New York was thinking about [building the tunnels]," Jonnes said. "The issue was money. The PRR was the biggest corporate entity in the United States. People in New York didn't care about getting back and forth."
Some 1,200 trains a day steamed into various New Jersey terminals. By 1901, the railroad ferries carried 80 million passengers a year.
To build tunnels under the river, "you had to be a company that wanted entre to New York," Jonnes said, "and the Pennsylvania Railroad was that."
Cassatt, who had homes in Haverford and Rittenhouse Square, was determined to span the one-mile-wide Hudson, long known to sailors as the North River.
He'd have to outwit, and outlast, corrupt Tammany politicians; assemble land in New York's Tenderloin district; endure bad press from newspaper mogul Hearst, and get through construction setbacks and worker deaths.
"I wanted to write about an American success, about a monumental project that everyone would be familiar with," Jonnes said. "You say Penn Station and everybody knows it.
"But what was really appealing to me is that Alexander Cassatt was unknown. He was famous then and his sister, Mary Cassatt, was unknown. Now, he's the brother of Mary Cassatt," the American impressionist painter.
During about 50 trips to the state archive in Harrisburg, Jonnes found the details of what was then the nation's biggest, most difficult and important civil engineering project. They were tucked away in thousands of boxes of PRR records.
Six documented the progress toward the 1910 opening of the tunnels and Penn Station, the great Doric temple to transportation. They contained important files labeled "New York Tunnels and Terminal Extension." "This was thrilling for a historian," Jonnes said. "I had everything I needed."
Like a detective, she re-traced Cassatt's every step as he nursed the project in New York or watched over it from his office at the PRR's Broad Street Station. "He was an unusual man, powerful yet modest and considerate," Jonnes said. "He tried to be an honorable employer."
Cassatt died Dec. 28, 1906, at his Rittenhouse Square mansion before his project was completed. The trains of the PRR and Long Island Railroad would not begin running between Manhattan and the mainland and Long Island for about four more years.
Still, Cassatt had done what no one else had - and hundreds of people stood in a steady rain at Rittenhouse Square, waiting to say goodbye to the railroad mastermind.
Some employees shared a story about the day the boss boarded the morning local at the Haverford Station on the Main Line for the commute to Philadelphia and had a run-in with a trainman who didn't know him.
The train made an unscheduled stop, but the flag man, who was to wave a red flag as company rules dictated, instead sat on the back of the car chewing tobacco.
"Cassatt asked if he wasn't supposed to be waving the flag," Jonnes said. "The flag man said, 'I don't know if it is any damned business of yours.' "
Cassatt retreated, muttering, "Certainly not, certainly not."
Later in his office at the Broad Street Station, Cassatt summoned the trainmaster, who vowed to immediately fire the worker.
"No, you won't fire him," Cassatt said, according to Jonnes. "But tell him not to be so disrespectful to people who ask for information in the future."
The man who would transform the lives of millions of people in water-locked Gotham simply wanted a little civility.
"The funeral was a very poignant episode," Jonnes said. "Cassatt was a noble man, and the coming of so many Pennsylvania Railroad employees to pay their respects was touching."
Contact staff writer Edward Colimore at 856-779-3833 or email@example.com. To comment, or to ask a question, go to http://go.philly.com/askcolimore.