Train
This little train functions by drawing power from the rails and delivering it to the motors. The train also features a small distance sensor, microcontroller, and speaker inside. If there is something in front of the train, it will let off a train whistle sound like a real train does to alert whatever is on the tracks.
Kids can interact with the train by changing its speed and placing items in front of it. What kid doesn’t like to see stuff smash into other stuff?
The train itself should cost around $15 due to microcontrollers, speakers, motors and plastic injection molding. The controller, essentially a variable DC power supply, will cost similarly due to PCB manufacturing and components. Finally, the rails will be inexpensive due to simple injection molded plastics and embedded metal.