A Rolling Road is a test setup for a locomotive that allows it to be tested without actually moving. Its driving wheels rest on and pick up power from ball bearing mounts. This is very useful for testing DCC decoder functions, especially sound.
In the days of steam, some shops had a length of test track coated with grease. A freshly shopped locomotive would be driven onto the test track, and the throttle opened up. Since there is little friction between the rail and drivers, the drivers slip. This allowed the shop crews to simulate miles of travel over a few minutes and a short distance. Adjustments could be made and quickly assessed in this manner.
One manufacturer is Bachrus Inc., which makes saddles to support the wheels, and other accessories. They also offer a digital speedometer, which can be used for break-in, trim adjustments, and speed matching locomotives. It is offered as a standalone unit, or as a USB interface to JMRI.
Bachmann make supports in two gauges (OO/HO and G) and in two varieties. Their E-Z Rider™ ball bearing ones are intended for locomotive driving wheels and the less expensive non ball bearing ones are for supporting undriven wheels (such as pony trucks and tender wheels). All are intended to rest on live track for full electrical pickup.