Basically, all parts of the layout are live at the same time, unlike DC where you have to switch sections on and off or from one controller to another. The "command station" sends instructions, in the form of a digital data stream, to the locos, each of which has a "decoder" or "chip" inside to receive its own instructions whilst ignoring the instructions for the other locos.
You can run multiple trains at the same time depending on the current capacity of your "booster". The booster amplifies the digital data from the command station so that the locos can also take their power from it. Boosters can go up to at least 10 Amps. Larger layouts may use multiple boosters for more current or to allow sections of layout to be isolated for fault finding.
You still need to be careful with return loops, just as you do with DC. An auto reverse unit can be used to handle the polarity switching automatically.
PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.