The correct answer is 15 km/h.
The speed of a train over points shall be 15 km/h when the points are clamped and padlocked after disconnection of points. This is to ensure the safety of the train and its passengers. When the points are clamped and padlocked, they are in a fixed position and cannot be moved. This means that the train will always travel over the same track, which reduces the risk of derailment.
The other options are incorrect because they are too high. A train should never travel at a speed of 20, 25, or 30 km/h over points that are clamped and padlocked. This is because the points are in a fixed position and cannot be moved, which increases the risk of derailment.
It is important to follow the speed limit when traveling over points. This will help to ensure the safety of the train and its passengers.