The answer very much depends on what is in your consist. Each handle of a Z4K can out put up to 10 amps at 18VAC. So a freight train with one engine and no smoke will draw far less than a double headed passenger train lit with incandescent bulbs and smoke unit on. This is why many of us are changing passenger car lighting to LEDs. Far less amperage required.
AFAIK, having the voltage drop a bit as the amperage increases is normal. Don't try to gang transformers together on a single track to get more amps! That's asking for trouble!
Chris
LVHR