It is a perfectly reasonable question because you are right...without some accommodation for the reversal of the rails, you will have a problem at one end of the bridge if you turn the engine 180 degrees. How virtually all modern scale TT's work is to have what is called a split rail that gets wiped by the bridge power leads. Each rail gets power for all but about 1/2" on either side of its full turn around 360 degrees. At that point, the two gaps, each rail goes dead briefly and the sound engines will cease emiting sound. Within a second you will hear the sound start up once more.
Each of these split rails is, of course, fixed in place. You have to take care to orient the two gaps away from the radials and the lead(s) to the TT. In the case of the indexed digital one by Walthers, there is an optical eye in a tiny aperture on the side wall of the bridge pit, and that, too, must not be placed under a lead or one of the radials/bay tracks.
It all makes sense once you begin to read the instructions and to place and orient the TT in your plan. I found it very straightforward. To conclude, though, the rails swap sides when you rotate the engine, but they end up wiping two fixed rails below them, each in a long arc. When the engine is turned, the rails under each side of the drivers end up with power oriented correctly for the approach to the radial or the lead as the case may be.
Some turntables have to be reversed like a reversing section and the rails have to be reversed using a DPDT or some other wiping device that effects a reverse. Or, of course, a digital reversing unit such as those made by MRC or Tony's.
-Crandell
The Atlas turntable automatically accounts for the reversal of polarity. If you look at an Atlas, you'll see that one group of stalls is labelled A, while the rest are labelled B. The polarity transition occurs between the stall groups. You'll notice a short power interruption as the turntable rotates and passes the transition point. It's done with mechanical wipers and split rings beneath the deck.
It takes an iron man to play with a toy iron horse.