Question does the 0v rail = ground, or no?
Short answer: maybe.
Point to consider: that doesn’t really matter if you are analyzing a DC power supply’s output for AC ripple, proper voltage level, etc.
Longer answer: “ground” is a bit of a misnomer of a term, because there are different types of “ground”. You have an actual ground or “earth” reference in a domestic, commercial or industrial power service, where the service panel has a ground buss bar physically strapped to the earth via a copper stake driven into the ground…it’s there to dissipate current that, due to a failure, goes where it’s not supposed to go and in order to avoid it going through you we have the “safety ground”…electricity, like water, always seeks the path of least resistance. Then devices have a chassis ground, a common reference for the device and this chassis ground may or may not be connected to “earth”…the chassis ground may “float” (and you can know if this is the case if the power cord has only two prongs, and not a third ground pin, and the chassis is not somehow otherwise connected to the service safety ground). Then you have DC power supply common references. And then you have circuit “ground” or common references, and this includes signal ground or shielding. This also may or may not be connected to the chassis. It depends on the device and the circuit. Usually it is. So regarding your power supply, and “0V”, think of it this way…a +15VDC supply is 15V right? 15V from what? The 15 volts DC is a power differential measured between two levels. You have to have a common reference from which to measure that power differential. We call that common reference the “0V” reference. It in and of itself is not a power rail…neither is the +15V. Together they make a complete circuit and comprise the power rail. It could be +13V and -2V. It’s still 15 volts. So, sure…call it a ground. Call it the common reference. Or call it the 0V reference because that’s what it is. This is even more relevant when there is a common 0V reference or null shared in a dual power supply, where you have +15V and -15V, for instance. The 0V reference is at the center of the 30V differential between the + and - outputs, and the current ”flows” through the electrons at each output, albeit in opposite directions, because of the bi-polarity in a dual supply. If we call it a “ground” that’s kind of a misnomer because “ground” comes from “earth” and that has to do with the safety ground in the electrical service, and, again, the chassis, power supply common reference(s) and circuit grounds in a device may or may not connect to “earth”…they may float instead. This is why, when measuring for things like AC ripple at the output of a regulated DC supply it’s important to measure between the 0V reference and the supply output. If you measure between the chassis (for instance) and the supply output, you may get false results if the 0V supply reference is not strapped to the chassis via a low or “zero” resistance path. You’re trying to measure for ripple across the supply differential. Not a supply output and a separate supply output…not a supply output and a signal output, not a supply output and some other supply’s 0V reference.
Hopefully that helps.