The Amtrak list of prohibited items includes the following:
Large sharp objects such as axes, ice picks and swords
Does this include small knives (such as a Victorinox SwissChamp, with a 2.5-inch long blade)?
Note that the prohibition linked to only mentions checked or unchecked luggage, and not items you might carry on your person.
I do not know this for a fact, but am guessing the rule is not influenced by the airline's prohibition on knives and other sharp objects after 9/11, but instead because trains are subject to quick, sudden jerks and jolts, both laterally and less often, front and back, and any large sharp objects such as those listed could rip through and even exit whatever they are packed in and present a hazard to others.
So a knife blade contained within a so-called Swiss Army Knife should not be prohibited.