Starting at "Zero"...

A resistor is a bad way to do it, consider some back-to-back diodes.  Each diode pair drops the voltage around .6-.7 volts.  In my experience, starting below around 2 volts is pointless, so if you have a transformer like the ZW that starts at 6V, you would need either six or seven diode pairs to drop the starting voltage to 2 volts.  A compact way to do the diode pairs is to use a bridge rectifier of the proper rating, each bridge rectifier will replace two sets of back-to-back diodes.  You need bridge rectifiers rated for the maximum current you'll be drawing from the transformer to the track.  Something like this 10A Bridge Rectifier bolted to an aluminum heatsink would be effective to most uses.

Here's two bridge rectifiers wired to drop AC voltage, note that this pair would drop around 2.8V to 3.2V.  If you need more voltage drop, add more bridge rectifiers.

Thanks, John.👍

Mark in Oregon

This can be accomplished easily with transformers like the 1033/1044 by using "B" as common instead of "A".

Like the diode method, this too will result in a usable voltage range of 0-11 volts instead of 5-16 volts.

That's why I like to start with something like the ZW, it normally tops out around 20 volts, so knocking off 4 volts with diodes still gives you 16 volts at full throttle, sufficient for almost anything.

I've accomplished the reduction by plugging a ZW and other postwar transformers into a 10A "variac" (autotransformer.)  I start with the variac on about 75/120.  The output of any transformer is proportional.  If I max out the handles I can always turn up the variac that feeds the primary.

One of the nice things about this approach is the tactile "feel."  You can feel the ZW rollers moving across the secondary, and you can feel the wiper on the variac moving across its windings too.  The Z-4000 is highly regarded, but it doesn't start at zero volts either.  And compared to the setup I described, the plasticy sliding feel of the handles is somehow unsatisfying.