Here's the deal. In a volume of water every molecule is connected to another such that all the water is connected. This one small sound reverberates throughout. But most important here is that when you pull the train stopper and the first molocules first flow into the drain then the remaining molecules fate is set... they all will go down.

This in the case, the amount of time required is directly proportional to the volume of the water and the size of the drain. Akin to a leaking dam. Once the first leak starts then in quick order the hole becomes larger and larger and the draining then becomes faster and faster.