Main parts

• An open model train car of type Nd (you may use basically any other car that fits you needs).
• Single-cell LiPol battery (500 mAh, 3.7 V).
• Axis with perforated wheel.
• GP1S23 opto sensor.
• Main PCB connected to sensor & battery.
• Bluetooth module HC-05 connected to main PCB.

Recommendations

• Use fixed coupling, the measured data will be much clearer.