There were 10 cabooses in all with each caboose having their own slogan, which was keyed to the last number. According to this plan, this one should be, "Look Around Getting Down".
The Seaboard Air Line Railroad was in operation from 1900 to 1967, when it merged with the Atlantic Coast Line Railroad. It then became the Seaboard Coast Line Railroad. In the '50's, the route mileage was a little more than 4,000 miles.
According to Wikipedia, Air line, in the days before air travel became a reality, was a common term for the shortest distance between two points: a straight line drawn through the air (or on a map), ignoring natural obstacles. Hence, a number of 19th century railroads used "air line" in their titles to suggest that their routes were shorter than those of competing roads.