Track Sensor
Track Sensor turns a signal key on while a car passes over it. It is the usual way to make something happen at an exact point on the ride.
It holds the key on for at least 4 ticks, so even a fast car triggers it properly.
Recipe
Where it goes
Place it on a rail with the Track Sensor item. It sits on any rail or anchor except a Station Rail or a Switch Rail, which have jobs of their own.
Giving it a key
The sensor has no menu of its own — you reach it through the block it sits on.
Use the wrench on the rail
Interact with the rail block carrying the sensor using the Coaster Wrench, not with the sensor.
Choose Track Sensor settings
The rail's menu lists its own options first, then the sensor's underneath.
Set the signal key
Name the key the sensor turns on while a car is over it.
On a Pro Rails track the sensor drops onto a plain Coaster Anchor and is reached the same way, through the anchor's menu.
What to wire it to
Pair it with a VFX Emitter or an SFX Emitter and you have a scare that fires exactly on cue. It also drives a Switch Rail or a Track Controller without a redstone line along the track.
See Redstone Signals for how keys travel.