[zabzonk]

These were all great. I have a question on the Tube train one though - as someone that has lived over 20 years of his life in London, a lot of them using the Tube, why don't I ever perceive actual Tube trains as going backwards? Or indeed ordinary mainline trains?

[Shraal]

I suspect that the illusion/gif is ambiguous because it's made of just two repeating frames. A "smooth" version of it with more frames would reveal the real direction of the train.

[arc-in-space]

Because you can see lot more than like... 3 frames or whatever it is that gif has; plus you have other cues, like the wind, sound, ground vibrations.