A 12VDC train power pack puts out 12VDC (in some cases more) when turned fully flat out. The loco will fly off the track before that happens. At normal operating speeds for the Showcase power unit, the power pack will probably be putting out no more than 3VDC at the track. Operating the power unit flat out for an extended period is impractical at best and careless at worst. In short, just because a power pack says it is 12VDC, doesn’t mean you have to run the train at 12VDC. There are two exceptions to this.

1. Pulse-Width-Modulated throttles
These throttles put out short bursts or spikes of voltage – reducing the gap between bursts as you increase the throttle to increase speed. Depending on the manufacturer, these can be 12VDC spikes. In the past I have had some Märklin Mini-Club iron-core motors get very hot is run for long periods of time at train shows using PWM throttles. I have not experienced that heating with the coreless motor Kenji uses. Some manufacturers such as Rokuhan and Varipulse have PWM throttles which are “kinder” to low voltage motors – both coreless and can-type.

2. DCC
DCC motor decoders are basically PWM throttles. The decoder can be programmed to get a better range of speed vs. throttle setting for these small motors. Bryan Vianco has a very good tutorial on setting the motor voltage CVs on his web site at tutorialstipstricks  "Setting Maximum Decoder Motor Voltage".   In addition to this, voltage can be reduced electronically using voltage reduction devices between the power unit and track.