As a train starts from rest then accelerates down the track, coming toward an observer faster and faster, the frequency of the sound waves coming toward the observer will be

Increase in frequency constantly

Explanation:

This concept can be explained by Doppler shift as per which any form of disturbance spreads out spherically from its point of origin. If the source of sound wave is stationary and the receiver is also stationary, then the frequency remains the same but if the source is moving, then the disturbance caused is closer on one side and distant on the other side thereby producing shorter wavelength in direction the source is moving and longer wavelength in the opposite direction.