Rail is a narrow pipe between two polygons (about the wheel's size) so that you can drive normally through the pipe. The head is outside the rail. Rails are used in rollercoaster levels. If you make the rail size smaller than wheel's size and have vsync switched on, you can use it to get more speed by pressing brake. See special level 1 in Playing section.
Refresh rate of your screen tells how many times the picture is drawn to the screen in a second. If vsync (vertical synchronization) is set on, the refresh rate limits FPS (frames per second, i.e. how many times in second the game situation is calculated).
See Tweaking page.
More info in Wikipedia.
Replay or shortly a rec file is a file where all the moves and locations of a single ride are saved. Since belma the replay name could have been up to 15 characters long (before belma only 8 characters). If you get an internal error when you save a rec, you probably don't have rec folder or it's write protected.
Rollercoaster is a kind of level which consist of rails and speedloops throughout the level and usually has only one route to finish. The concept was invented by px and since then every world cup have has a rollercoaster level, usually made by px.
You can rotate/volt your bike by using left and right arrow keys. Early beta versions of Across didn't have rotate keys at all, which means that basically every level could be finished without them. And actually they can, see 8-ball's crippled wrs (no volt). At least Markku has finished all the levels without rotate keys. By pressing both arrow keys exactly at same time, you can get a supervolt (see supervolt). For some volting levels, see Playing section.