A little history here might help clear up the DCC voltage to the rails issues on several different levels.
At one point in time many years ago the NMRA Standards and Recommended Practices did include voltage to the rail ratings for the scales of N, HO, O/S, and G. Over the last 10-15 years the NMRA has modified and/or deleted many Standards and Recommended Practices. The DCC Standards and Recommended Practices are a perfect example. Several of the DCC Standards have been modified and most of the Recommended Practices have been deleted. Unless you know where to look finding the older Standards and Recommended Practices are very difficult. I have since DCC was first introduced back in 1994 tried to keep up with any and all information dealing with DCC. It has not always been an easy task. I have a complete set of the DCC Standards and Recommended Practices as they were back in 2002.
In the latest Electrical Standard S-9.1 (Version 2006), on page 3, there are four charts which indicated the various voltages of the different scales for DCC. If you are curious about the voltages please follow this link:
Now lets take a minute to look at the DCC Decoders. If one were take a close look one would find that any DCC decoder is supposed to accept an input range anywhere from about 12 volts to 22 volts. This holds true for most non sound mobile decoders. Sound decoders in general cannot handle voltages above, I believe, about 18 volts. This is a component issue dealing with sound chips. But all things being equal it is the motor in the scale that determines what voltage is needed on the rails.
Now as for the voltage output to the rails the issue deals with the market place. HO has been the most popular scale for at least 30-40 years and HO layouts have a history of using 0-12 volts DC for operation. With HO being by far the most popular scale the manufacturers lean in the direction of HO for most of their support. Just look at Walthers. Walthers today only produces two general catalogs: O and HO-N-Z. It was only a couple of years ago that Walthers produced three catalogs: O, HO, and N/Z. The number of products in HO in their HO-N-Z catalog is much larger than their N scale products which is much larger than their Z scale products.
DCC manufacturers are in the business of making a profit and thus they place their major support to the market with the greatest potential for profit which is the HO market. Thus most of the DCC command systems are HO scale based when it comes to voltage to the rails. There is a general drop of about 1.4-1.8 volts through a decoder thus in order to get the 12 volts to the motor the voltage on the rails must be at least 13.8 volts.
It is easily understandable why most of the DCC command system manufacturers have only one voltage to the rails output and that is most of the purchasers of their command stations are HO scale modelers. Besides N and Z being much smaller markets when it comes to model railroading note that N and Z scale decoders are more expensive because the parts need to be smaller and also note that it is much more difficult to install a decoder in a N and Z scale model railroad engine. I have a HO 0-4-0 yard switcher that is over 35 years old and it was easy to install a N scale decoder in it once I isolated the motor from the frame. It appears to me that over the last several years there has been a general drop in the number of model railroaders in the scales of O/S and above.
The only DCC manufacturer that I am acquainted with that provides easy to adjust voltage to the rails is Digitrax. All of their command stations with the exception of the Zephyr have an external voltage to the rails adjustment switch for N-HO-O/G with the output voltage dependent upon the command station power source. The current PS615 is great for HO and below while the PS2012e can easily handle all scales to include the scales above HO. Digitrax a couple of years ago added to their product line the UP6Z which can easily adjust the voltage to the rails for Z scale operation to about 8 volts.
The above paragraph is not intended to indicate that Digitrax is any better than any other DCC command system but only that the Digitrax hardware is geared to handle a wider variety of model railroading scales. Each DCC command system manufacturer has chosen which market it wants to support. When choosing a DCC command station or any other DCC related product one must make the decision that best fits his, or her, situation. I operate on several different layouts from time to time and I enjoy every moment no matter what the scale or which command system is being used.
The one aspect of DCC that I appreciate the most is the ability to move with my engine without the need of having to keep track of the rail power routing switches.