The locomotive you're using is it PS2 5V? There have been some issues with PS2 5V locomotives and the Explorer. For some reason they don't like it, but it's not every PS2 5V locomotive. I've seen one PS2 5V (a RK 0-8-0) that just wouldn't work with the Explorer no matter what we tried. Worked fine with a full DCS system and a remote commander though.