Reading track voltage correctly on DCC can prove to be a challenge. The voltage available on the rails is a Digital PWM form of AC, not standard AC.
For this reason often times a regular meter will not give accurate readings on the rails or on the two-pin plug out on the back of the Powercab panel.
However, the Powercab Power Supply output is Standard DC. The power supply that was included with your Powercab may be one of several versions based on what was available to us at the time. The question is the voltage and current ratings of the actual power supply you have. Printed on the power supply you should see a voltage rating and a current rating. Something like - 12v dc or 13.8v dc. and 1.35A, 1.44A, or 1.8A current rating.
If you have a 12v power supply, I would consider the 10.8v reading on the rails to be correct given all the possible variables.
If you do not own a multimeter here is a link to an inexpesive one from Home Depot - http://www.homedepot.com/p/Powerbuilt-Digital-Multi-Meter-648349/204505225