I.over [美 ˈoʊvər, 英 ˈəʊvə]副over often appears as the second element of certain verb structures in English (blow over, knock over, pore over, etc). For translations, see the relevant verb entry (blow, knock, pore, etc).
It does this by inflating itself by filling its interior with water and then emitting jets of water on one side to make the whole structure topple over.
The initial motion of the wheel beginning to topple over is equivalent to applying a force to it at 12 o'clock in the direction of the unsupported side.
At the same time a down express passed through the station and collided with the derailed wagons at speed causing the front of the express to derail and topple over.