The Heathkit transformer I’m thinking of was designed with HO trains in mind (single-motor, open frame or can, DC) from the 1960s and ‘70s.
The max amperage draw of the typical HO locomotive of that era would have under 1 amp and, more importantly, would have responded more aptly to a lower voltage setting, operating realistically at less than 12 volts, thus allowing a higher amperage draw within the VA rating.
Your AF locomotive probably runs at close to 15 to 16 volts and also demands more amps at that voltage. My O gauge locomotives with dual can motors can draw between 2 and 3 amps at normal speeds and under load from my MTH Z-4000 transformer, which displays both the applied voltage and amperage.
You can’t simply use any DC power source to run a universal motor, and you always need to determine the amperage requirements in any electrical application.
