The correct answer is: A. 1.23 m above the rail level.
The centrifugal force due to curvature of track is assumed to act on the bridge at a height of 1.23 m above the rail level. This is because the centrifugal force is proportional to the square of the speed of the train and the radius of curvature of the track. The speed of the train is constant, so the centrifugal force is only affected by the radius of curvature of the track. The radius of curvature of the track is the distance from the center of the track to the point where the train is traveling. The closer the train is to the center of the track, the smaller the radius of curvature and the larger the centrifugal force. The centrifugal force is assumed to act at a height of 1.23 m above the rail level because this is the height at which the train is closest to the center of the track.
Option B is incorrect because it is the height of the rail level. The centrifugal force is not assumed to act at the height of the rail level.
Option C is incorrect because it is the height of the bridge. The centrifugal force is not assumed to act at the height of the bridge.
Option D is incorrect because it is the height of the center of the track. The centrifugal force is not assumed to act at the height of the center of the track.