Does one need to identify himself to travel with train in USA?

Here in Russia we need to present our passports in order to buy tickets on airplanes and trains. For example, I can't buy ticket for my friend.

Are americans need to do this too?

yep

especially after 9/11

well you can buy ticket for a friend but his name will
be on the ticket and your friend will have to show some id
(passport)_just make sure the ticket wasnt stloen or something

I commute each day on CalTrain, Amtrak's Capitol Corridor and ACE here in California, and have NEVER been asked for my ID.

Amtrak does require that you have valid ID on you at all times: http://www.amtrak.com/servlet/ContentServer?pagename=Amtrak/am2Copy/Title_Image_Copy_Page&c=am2Copy&cid=1080080554204& ssid=342

No, not yet

Yes. If buying a ticket using a credit card, the ticket must be signed by the purchaser before use. I bought a ticket for cash today on the Capitols train from Richmond, California, and the conductor asked to see my identity document showing my photograph.