A portable advertising system well suited for outdoor advertising at night includes a body adapted to be secured to a transportation vehicle and may include a universal attachment system for that purpose. An advertisement may be displayed on an advertising surface mounted on the body. An illumination source is directed towards the advertising surface and may be integrated into an electronic display unit or may be directed at an advertising sheet such as a vinyl sheet. When the advertising sheet is used, a sheet tension system typically maintains tension on the sheet, and an openable protective cover may protect the sheet when not in use. The illumination source may be controlled based on environmental light conditions, sheet tension and/or position of the protective cover. The body may be part of a cargo container such as an ISO container suited for securing to a railroad car or other vehicle.