Hello again PJ, you asked "Is there a way of knowing if the layout requires you to have SP1 before you download it ?" In short, I'm afraid the answer is no. There is no facility on the DLS to help us non-SP1 TRS2006 trainzers in being able to sift through layouts built by creators using SP1, AT THE MOMENT. I am personally hoping that the Auran heirarchy, may help us out in the future by implementing such a scheme, but at the moment you are on your own buddy. I will not name names, but there are a lot of clues given by forum members who are prolific route creators as to who is now using SP1 and as such I now steer well clear of their uploaded routes for TRS2006 as I know that I personally will never be able to play them, which is a huge pity, as they nearly all look excellent layouts, and the man-hours spent creating them has been somewhat wasted if not EVERBODY using TRS2006 is able to download them. Of course, there are hundreds of routes pre-SP1 on TRS2006 on the DLS and all of those for TRS2004 and UTC, which are downloadable and offer you loads of suitable alternatives.
Cheers. ex-railwayman.