That's not where her mind was at the time.
She did not plan it. Her instincts just took over.
She did not say to herself, "I'm going to put myself between my son
and this oncoming train, even though I know that it will not save him."
It's just that standing aside and doing nothing was not an option.
She was not thinking of herself or the personal consequences at all.

Dumbledore, on the other hand, had a year to think about it.
A year to realize that it would be pointless.
And that faking his death would do more good than the real thing.