AMTRAKS are excellent...for what they were designed for. They are designed to transport Marines from ship to shore with a modicum of protection from shrapnel and small arms. They have to float. They not only have to float by themselves but with (iirc) 28 fully combat loaded US Marine Infantry on board. I have no idea what the average weights are but I was generally in the 200lbs range and could have as much as 180lbs of
gear on and I was pretty average.
They were never designed nor intended for extended land war use. This is the USMC tho. They will use what they have to do what needs be done and will accomplish the mission regardless.
The USMC is a light infantry shock troop type organization. They are not mechanized to any real degree.