because that transformer puts out less voltage to start with even a zw only puts out 20 volts if I remember so the lower wattage also enters into the picture ,the lower the wattage the quicker youll run out of enough voltage to run old post war trains draw more power then todays train, . a KW I believe if I remember correctly puts out 25 volts! the z4000 transformer puts out 22 volts at 400 watts I believe!
you get what you pay for.
