Set the transformer to 18 volts.
See here, page 5
https://www.lionelsupport.com/media/servicedocuments/6609730233250PAFlyerRC.pdf

Larry

You're welcome.
Enjoy your trains.

Larry

Chief,
Thank you very much, that question was also on my mind.
Chuck

Regarding lowering AC voltage using a transformer:  Attached is a chart of maximum speeds (in scale miles per hour) for the Diesel from a 6-84719 Santa Fe Super Chief set and an 0-8-0 Steam Engine from a 6-84725 Southern Pacific Rising Sun set.  They are somewhat similar behavior from 14 VAC to 18 VAC with perhaps 16 VAC being the "sweet spot" for a nice top speed of 60 mph.

Also, I did current draw tests under different conditions for the two engines.  Running at a moderate speed, the Diesel was 300 mA and the Steam engine 150 mA.  Stalled at a moderate speed setting the Diesel was 700 mA and the Steam engine 450 mA.  Stalled at the maximum speed setting the Diesel was 1500 mA and the Steam engine 1620 mA.  Since the 36 watt "Wallpack" that comes with each set is 2 amps, one can easily handle both trains unless both were stalled at maximum speed settings.

If you want to use the LionChief 36 watt Wallpack (that comes with each set) and drop its voltage, there is a way.  The Wallpack is DC and a cheap and easy way to reduce its output voltage is to wire a string of diodes in series.  Each diode will drop about 0.6 volts (and dissipate 1.2 watts if the supply were delivering its full 2 amps - typically much less).  Just use as many diodes in series as you need to drop the maximum speed to something less terrifying than the stock maximum speed.  And use diodes with at least a 3 amp rating.