Auran/N3V has recently released a new service pack with fixes for Trainz 2012. Improvements, on paper, include better working splines, improved multiplayer and sessions; in-game means of downloading; better physics and a slightly improved CMP.
However, from what I hear it's a buggy service pack. It's causing some content to derail, and most importantly, some Jointed Rail content doesn't work.
Auran identified the derailment issue, and Jointed Rail has provided content hotfixes for the SP1 glitches. (more information http://jointedrail.com/content/updates/JRSP1Updates.pdf)
My two cents. I feel that a service pack shouldn't be downloaded until all of the glitches are weighed. Upgrading the game is generally a choice for the owner. I feel that while there needs to be people testing the ropes of the pack, it is wiser to hold back and see how much reception the pack gets. Technically upgrading can run the risk of having to re-install the game in the unlikely event your machine fails to boot up the game.
Some people in my Skype chat are having quite a number of issues. But, I really hope the issues get rectified and upgrading can be smooth. I am nonetheless looking forward to the improved features that SP1 is out to deliver.
