Is this train coming into or leaving the Charing Cross station? Focus on it and you’ll be able to change its direction! (1) 

The original movie

This train is coming into Charing Cross:

The movie is actually a 4-frame animated GIF:

Reversing the frames

By reversing the 4 frames, the train leaves the Charing Cross:

See here the 4 reversed frames:

Creating an ambiguous movie

The ambiguous movie can be created just swapping the two central frames:

The result can be seen on top.

Hanging the focus

Focusing on to the two distinct directions can be difficult; in order to help focusing on both, I have added two arrows pointing at two balls – one red, one yellow:

Slowing its speed, it is easier to appreciate the resulting ambiguity:

Notes