How do i measure the maximum current of an AC variable power supply with resetib

it's a lionel train trainsformer
says:

  • 7.5-V.A.
  • 8-15Volts
  • !!!AC current only!!!

my main question is this...
The 7.5Volt.Amps. is for what voltage? 8V? 15? Both?
and if it is for only one voltage level, what is the maximum Volt.Amps for the other & what will the maximum V.A. be at differant Voltage levels between 8 &15?