LONG BEFORE THEY famously debated each other during the 1960 presidential campaign, John F. Kennedy and Richard M. Nixon debated the merits of the new Taft-Hartley labor law in McKeesport, Pennsylvania, in April 1947. But their minds were clearly on bigger things. As fate would have it, Kennedy and Nixon shared a Pullman compartment on a famous train called The Capitol Limited, the pride of the B&O line, for an overnight trip back to Washington. They...