Okay, what if, like, it was your dog and Hitler, which would you save? BUT it's actually a young Hitler so he hasn't done anything bad yet. Also, if you don't save Hitler, he is sucked up through a vortex just before being hit by the train and becomes bitter, thus leading to the Holocaust. And if you save your dog, he eventually goes on to randomly attack one of the 9/11 hijackers at an airport, thus exposing the plot and keeping the Twin Towers from being destroyed [and also to George Bush being a little known one-term President, which also means no war in Afghanistan or Iraq].
So...dog or person?