Conestoga wagon

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A heavy covered wagon with large rear wheels, used especially by American pioneers as they traveled west.

from the GNU version of the Collaborative International Dictionary of English.

• A kind of large broad-wheeled wagon, usually covered, for traveling in soft soil and on prairies.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A large cart with a canvas cover drawn by mules, oxen or horses and used extensively during the Westward Expansion of the United States in the 18th and 19th centuries.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a large wagon with broad wheels and an arched canvas top; used by the United States pioneers to cross the prairies in the 19th century