the train will depart when i tell it to.请教下这里to的用法。

这里的后半句为省略句，完整的可以说：the train will depart when i tell it to depart.

当这里的省略的前提是句中的两个动作发出者是同一个

e.g.   Tom wanted to touch the dress bu his mother told him not to.

不定式的省略：保留to省略do动词。

　　If you don't want to do it, you don't need to. 如果你不想做这件事，你就不必做。