Intermodal containers are divided into two groups: domestic and international. The former are owned largely by domestic providers like Union Pacific (NYSE: UNP) and JB Hunt (NASDAQ: JBHT) and remain largely on the North American continent. International containers are the ones you read about being in short supply and owned largely by maritime providers like Maersk. These two container types represent distinct transportation sectors with very different issues.
It seems that somewhere along the line, shippers and transportation providers have figured out how to transload more of the international containers (mostly 20-foot and 40-foot) to the larger domestic versions (48-foot and 53-foot) and get them moving across the country.