US to South America

Some freight lines take on a handful of passengers on a freight steamer. I used to go down to the docks and ask the captains to take me stateside from Santos in Brazil.

Oh wow. That sounds really cool! Thanks!
Any other ideas would be much appreciated!

Also does anyone know if is there a bus or train service from the US into Mexico or anywhere else in South America?

Any Greyhound bus will take you a to US-Mexico border city, in which you take a Mexican bus and move south.
There are several classes of buses in Mexico. Prices, confort and safety vary accordingly, and differences may be steep. The passenger railroad system is worthless, so I wouldn't reccomend it.

BTW, Mexico is not South America. South America starts near the Panama-Colombia border, after you Cross the Gulf of Darién.
Central America, geographically, starts in the Ithsmus of Tehuantepec. Only 1 and 1/4 of the 32 Mexican States are in Central America; the rest is North America.
Call any Mexican (or a Cuban, or a Dominican) "South American" or "Central American", and you'll get a dirty look. We are Latin Americans, which is not the same.

Ok thanks a lot for the help! and the warning! So Latin Americans... ok cool. I am obviously not well educated on the subject. But thanks again for ur help!

Where in South America are you going to?
I sense you're going to Columbia or Argentina

Remember that Mexico is in North America.
Regards
Xavier