Accurate location information is absolutely vital for communications-based train control (CBTC) systems to ensure safe train running. For the present purposes, a high-precision approach to train positioning is proposed in this paper using optical camera communication and monocular vision measurement, which could present an effective solution to train positioning with high precision, low cost, low maintenance and anti-interference. The images of light emitting diode (LED) lamps are captured by a monocular camera, and the three-dimensional attitude angle of camera is measured by an inertial measurement unit. The coordinates and shape information of LED lamps are obtained by optical camera communication technology. The relative coordinates of the camera and LED lamps are acquired through the imaging principle and geometric relationship to realize the train positioning. According to the real-life line data and train parameters, experimental studies suggest that the maximum error of train positioning in the proposed method is 35.56 cm, the minimum is 1.78 cm, and the average is 12.26 cm, which can meet the requirements of train positioning accuracy for CBTC systems.