About four years or so ago, Cummins introduced its new heavy duty class 8 application Signature 600 engine, which is currently called the ISX. The prototypes were installed in about a half dozen chosen rigs which would pull those trailers, which were loaded to the max GVW, around the country to truck and trade shows. Designed to be a 600 hp max output, fuel economy leader, the signature 600 used a DOHC system and integrated a 6-cylinder selectable integrated engine brake system called intebrake. Upon having to bow to the new emissions requirements, the engines received cooled EGR, and the current maximum rating is 565 hp for the ISX in heavy duty trucks, 525 in fire trucks. I'd venture a guess that the trailer you saw was a fugitive from the new trailer logos.