I just saw a brochure for the Richmond Main Street Station, and it says the Seaboard Air Line was named after the train's air cushioned suspension. That is not true, is it?

Extremely unlikely.

Use of the term "Air Line" in a railroad's corporate name (New York & New England Air Line, Atlanta & Charlotte Air Line) was usually meant to convey that the railroad was laid straight and flat, the shortest and fastest path between the two points.

In the specific case of SAL:

http://railga.com/sal.html

"Seaboard Air Line Railway was created in the 1880s by the consolidation of the Seaboard and Roanoke Railroad and other lines in the Carolinas into a single system.

On July 1, 1900, the entire system was consolidated and reorganized as the Seaboard Air Line Railway, a 2600-mile network stretching from Virginia to Florida. The Seaboard Air Line name had been used previously, but as a marketing name rather than the name of the company."

JAC