Here is an album of 52 pictures of Amtraks. There were a number of different versions, but the basic concept is a tracked vehicle--angled at both ends--usually with a ramp for vehicles or soldiers. Here is one that's carrying a jeep. The early versions were open topped, while later versions added a roof for protection. Most were lightly armed with a machine gun or two. Here is an open topped Amtrak in the water. Wikipedia lists the number of different kinds of Amtraks used, and you can see that there were a bunch of different types. In that respect, an "Amtrak" is a bit vague as a description--it's more of a category than one particular vehicle.