This device is designed to measure the scale speed of locomotives or train cars that past by it. Two small light sensors mounted discretely under the rails are used to record the speed of anything passing over them and then the device calculates and displays the corresponding scale speed on a 4 digit display. 

The Trainspeed4 includes everything necessary to monitor train speeds at 4 different locations on your layout.

The display on this unit measures (WxHxD) 1.6"x 0.5" x 0.28" (40.7mm x 12.9mm x 7.1mm)