All aboard on this semi-sunny Wednesday.

There’s one train show at Grand Central Terminal and another at the New York Botanical Garden. Vintage subway cars are making special runs, model trains are zipping through Macy’s Santaland, and tales of the Polar Express abound.

Trains seem to be inextricably linked to the holidays. Why is that?

Christian Wolmar, a railway historian and author of “The Great Railroad Revolution: The History of Trains in America,” told us that the holidays were, in essence, “invented by the railways.”

“Until they had railways, people couldn’t travel very much,” he said. “There’s a strong association with time off or holiday time and the railways, because the two developed in tandem.”

The railways, which originated in the United Kingdom, existed for at least half a century before people could travel by car, Mr. Wolmar explained, so the association between leisure and locomotives became fixed early on. The British “railway mania” of the 1840s picked up speed in the United States over the few decades that followed, but trains here had their holiday travel “heyday” in the 1920s and 1930s, Mr. Wolmar said, “before the luxury market was taken away by airplanes and the mass market was taken away by cars.”