SP stands for "Service Pack" and usually contains fixes for bugs, (patches), even new content can be included.....you have to realize as the programs get more advanced such as Tane vs. TRS2006. more problems can occur and not to mention all the different pc configs available now vs when TRS2006 cam out and you can see why service packs are needed..........bottom line regardless how much testing can be done on a product prior to release there are bound to be some type of issues that got passed the testing phase and are discovered after the release. More features, better graphics, Advanced train AI, you can see where this is all going.............