function rollOverObject(name) {
    this.name    = name;
    this.off     = new Image ();
    this.off.src = "images/nav/" + name + ".jpg";
    this.on      = new Image ();
    this.on.src  = "images/nav/" + name + "_over.jpg";
    }

contact = new rollOverObject("contact");
beams = new rollOverObject("beams");
trailers = new rollOverObject("trailers");
equipment = new rollOverObject("equipment");
navlocation = new rollOverObject("navlocation");
bogies = new rollOverObject("bogies");
index = new rollOverObject("index");

function rollOn(imageID) {
  document.images[imageID].src = eval(imageID + ".on.src");
}

function rollOff(imageID) {
  document.images[imageID].src = eval(imageID + ".off.src");
}