In the train example, you always opt for the track with the most people on it...because it is best to crush your enemies, have them driven before you, and hear the lamentation of their women.
As far as the evilness of the actions regarding the time traveler and the evil person as an infant (avoiding all the time paradox stuff)- you are killing an infant, a person that has not made the choices that led you back in time to kill them. You would only be justified in that killing if they had reached the point when they turned evil- before that you have killed an inocent.
You cannot avoid time paradox in time travel.
With time-travel, the evil is a matter of perspective: To those who cannot travel time, you are killing someone who has not comitted an evil act. To those who can, you are killing someone who has- the actions of their lives are part of your history, after all.
However, the option no one ever considers in these examples is this: alter that monster's life without killing him. You don't kill Baby Hitler, you find out what caused his anti-semetism and try to alter
that...or simply relocate him.
In a sense, it is like the penalty for attempted murder. People get arrested for attempted murder every day, but in no jurisdiction of which I am aware is death the penalty for attempted murder. At worst, is is punished as non-capital murder- 25 to life (unless the crime is only "attempted murder" because the would-be killer is stopped in the act by deadly force).
Of course, the change has to be significant. Merely moving Baby Hitler into the home of Rabbi Feinbaum may not be enough. You might just be providing the catalyst for his anti-semetic vies when the Rabbi gets arrested for kidnapping him...
If, OTOH, Rabbi Feinbaum lives in the USA, or you t-port him to the doorstep of a mission in Africa...