Just to clarify a bit further (at the risk of boring most)...
If the range of the bipolar DC is +12V to -12V (24V peak-to-peak), the rectified voltage will be 12V. Note that it doesn't matter what the pulse widths are, the rectified voltage will always be constant since the voltage that the rectifier sees will always be either +12V or -12V. A rectifier doesn't care about pulse widths or pulse duration - it just changes the + or - V into a positive voltage level. There will always be either a +12 or -12 voltage present, so the rectifier will generate a constant 12V level, subject to further notes below.
The power fed to motors, etc., will use 0V as the common or "ground" voltage and will use 12V as the maximum positive voltage.
Due to ineffencies in rectification, though, the motor will see less than the max voltage. To compensate, the DCC system will usually operate at a voltage slightly higher than the target motor voltage. For example, if 12V is the target voltage for motors, the system may actually run at something like 25.2V peak-to-peak (12.6V positive and 12.6V negative). By DCC standards, the voltage can actually be as high as 27V peak-to-peak.
This means, though, that if any slotcar system exceeds 27V peak-to-peak, or approximately 12.9V DC for motors, then it is non-standard by DCC conventions.
Not sure how this translates to Scaley's version. If they are following true DCC conventions though, they can't exceed this voltage level. To feed motors at 15V would require a peak-to-peak DCC voltage of approximately 31.2V, outside of the current DCC standards limit.
As an aside, the DCC convention is one of the reasons why digital control is not currently considered appropriate for large numbers of devices (cars) where rapid changes in instructions are relevant. Each car receives its instructions via "packets" of data sent from the DCC source. Each decoder recognizes it's own set of instructions. Thus, there are a finite number of "data packets" that can be sent each second and this information cannot overlap. Each car sees instructions at intervals, not constantly. It's entirely possible that if a large number of cars are involved, that a given car may not see every input that its driver initiates. Consider how fast a driver can change trigger position - it would be desirable to send a packet to the decoder for ANY change in trigger position. If too many cars are on the system though, some of those inputs may not get sent. Believe it or not, the bandwidth of DCC has practical limitations. It works well for trains, but slot cars depend on much faster, and more frequent, changes in inputs.
Ooops... getting carried away, I fear....