R.
Radius : The size of a curve measured from its center point to the center line of a curved track.
Rail Code : The height of model rail in thousandths of an inch: code 100 measures .100" tall, code 83 is .083" tall, code 70 is .070", and code 55 is .055."
Railhead : The wider, top portion of a rail that the wheels run on.
Rectifier : An electronic device which converts AC current to DC.
Relay : An electromagnetic switching device commonly used to control polarity, power, and signal circuits.
Resistor : Restricts the flow of electrical current to lower its voltage or limit its current.
Return : The place where a connection is made to the track to allow current to flow back to the controller. In model railway terms this is called the negative.
Reverse Or "S" Curve : Opposing curves connected end to end.
Reverse Loop : A reverse loop is created where a track leaves a circuit and then rejoins it with the train going in the opposite direction. This will result in a short circuit unless the section is isolated from the rest of the circuit. A DPDT switch is then used to reverse the polarity of the circuit concerned.
Riser : A vertical piece in the benchwork that supports a track board or subroadbed.
Roundhouse : An enginehouse like a sector of a circle in shape, and usually surrounding a turntable.
Ruling Grade : The maximum grade that controls train size on a given route.