VA means volts times amps. A power pack rated at 25 VA means 2.5 amps at 10 volts, or any combination of numbers that multiply to 25. That means if the loco is drawing 5 amps (highly unlikely), then the max voltage the power pack will put out is 5. Conversly if the loco draws 1 amp, the power pack theortically could support 25 volts but probably has circuitry to limit it to about 12.
The amperage draw from any loco/item is what determines the required power pack/breaker rating. The heathkit at 1.5 amps is a pretty lightweight power pack. It probably will work pretty well with a lot of more modern locos with more efficient motors but not so well with older stuff, espically multiple units.