Ideal for block detection, block signal control and level crossings.
You get 6 IR sensors plus 6 pair of IR LEDs that you use to replace the standard 5 mm IR LEDs that are on the IR sensors.
The 5mm LEDs will not fit between the rails of OO or HO scale track.. 3mm LEDs will.
Getting the replacement LEDs saves you time and money by getting everything ordered at one time and paying one shipping cost.
And to make it easy to install the sensors and adjust sensitivity, install the LEDs remote from the sensors. You can see how to do that here.