There are great difficulties to test switch rail surface because of its continuous variable cross-section. A card-type eddy current sensor was developed, and used to test the narrow space located at the inner surface of switch rail. The sensor comprises an elastic non-metallic sheet, multiple detection coils and a single thin magnet. It fits well with the switch rail for its magnet sheet to reduce lift-off effect, and the detection sensitivity is improved. The thickness of the sensor is only 0.8mm, and can be used to test the inner surface defects of turnout between switch rail and stock rail. The experimental results show that the sensor meets the testing demands of the switch rails.