Today I called Amtrak to find out more about Amtrak Express shipping service. Apparently, there is an Amtrak shipping booth at every Amtrak Station, where you can drop off packages, and it gets delivered to any other Amtrak Station by freight. It's pretty affordable.
For Boston South Station to Oakland California:
$.57 each lbs over 100
500lbs $295 max
delivery in 4-7 days
Train also emits the least CO2 at 0.055lbs CO2 per Ton-Mile. Plane is a whopping 3.366lbs CO2 per Ton-Mile, and car is about 0.654lbs CO2 per Ton-Mile.
I've been trying to do my own little calculations for transporting things in my particular car. The easiest way to approximate how much extra emissions it takes to transport an extra 100lbs is to compare the average fuel economies with and without the extra weight. With just me, my car can average 33mpg if I'm driving on the highway 80% of the time. With an extra 100lbs, which is just a bit less than another person, I estimate that my fuel economy gets down to 28mpg at most. It takes an extra 5.4 gallons to travel 1000 miles with the extra 100lbs. There is about 19.4lbs of CO2 per gallon of gas. It comes out to being about 1lb of CO2 per Ton-Mile. The cost in extra gas would be about $64 for $4/gallon gas, so about the same as Amtrak Express.
(An optimistic estimate would be 30mpg, which comes out to 3 gallons more, which is about .582lbs per Ton-Mile and $36.)