As for Big John, I have no idea

Good luck

Andres

the first one is to transport liquids
the second one usually used to transport grains. They are open or covered, with the floor sloping from the ends and sides to one or more hoppers, which will discharge its entire load by gravity through the hopper doors.
And for the other one I've got no idea

From my research, a "Big John" car is a type of hopper car:

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FREIGHT CAR COVERED HOPPER / SOUTHERN RY. / 4713 cu.ft. #8000-8074
MAGOR CAR "BIG JOHN"
JIM SIX PROTOTYPE PHOTO SR #8055
Model Railroading Sep 1997 Page 50 Vol 27 Num 7
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http://www.sourwy.railfan.net/newpage1.htm
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The Southern Railway was an innovator in the railroad business. Among their firsts, were complete dieselization, use of computers, modern hump yard design, and innovative freight car design (all door boxcars and the Big John and Super Big John aluminum covered hoppers and many others).

http://www.garlic.com/~tomd/southern.html
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SR develops oversized grain cars holding up to 4,000 bushels. These efficient cars (named "Big John," after a popular song of the day) saved Southeast grain customers millions of dollars and were at the center of a celebrated legal fight for deregulated rail rates.

http://www.nscorp.com/nscorp/html/heritage/timeline.html
====================
In what would revolutionize the rail market for grain traffic, Southern Railway, an NS predecessor line, in 1962 developed an oversize grain car known as Big John. The idea came out of Southern's determination to offer the best possible rates to customers. D.W. Brosnan, president of Southern, told Stanley Crane, then director Industrial Engineering, to build "the biggest car you can to haul grain." With the help of Reynolds Aluminum engineers, Crane and others from Southern designed a car with 5,000-cubic-foot capacity. With four compartments, the new grain cars could carry up to 4,000 bushels of grain. Brosnan added the name "Big John" when an Indiana newspaper reporter suggested the cars be named after a popular song of the day.

http://www.nscorp.com/nscorp/html/update/iu08-14-98.html
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Here's info on hopper cars:

Hopper
An open top car with hinged trap doors and inclined floors which permits quick unloading of bulk commodities.

http://www.ksry.com/terms.htm
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And this is what I found over at EuroDicAutom:

hopper wagon = vagón de descarga por el fondo (BTL-UIC75-0011551)

hopper wagon, hopper car = vagón de descarga automática (BTB-TDC77-0086104)

hopper wagon with opening roof = vagón tolva con techo corredizo (BTL-UIC89-0010171) -- On this one, I'd check with the client and see what kind of roof these cars have, since it may not be a sliding roof (or is that the only kind?).

EuroDicAutom
http://eurodic.ip.lu/cgi-bin/edicbin/EuroDicWWW.pl

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I don't have a Duden pictorial dictionary handy, but if I remember correctly, they have a fairly good section on railroad terms. Other pictorial dictionaries should help as well.

Hope that helps! And good luck!

Terry @ Intergraf Americas