This site started off life as several unrelated pages across two servers, with cross-connected links between the two. That is not a really good way to do things, and having sufficient space on one of the servers for a number of decent sized images, the current format was developed. A "framed" site was chosen because it is so much simpler to use a seperate side-bar menu to go from page to page without losing continuity.
Another answer relates to why the need. It was felt that a central reference point for the railways within South Australia could be useful, if nothing else to provide a series of links to the individual operating companies. This has been done, some way down the menu bar on your right.