switchback

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

• noun A sharp bend in a road or trail on a steep incline.
• noun A point on a mountain railway where trains reverse direction to continue obliquely up or down a slope without turning.
• noun A road, trail, or railway that has such bends or points in it.
• noun A section of such a road, trail, or railroad, extending in a nearly opposite direction to the section coming before or after it.
• noun Chiefly British A roller coaster.
• intransitive verb To proceed in sharp turns in alternating directions on a steep incline.

from The Century Dictionary.

• Characterized by alternate motion, or by motion back and forth; pertaining to or adapted to use on a switchback: as, a switchback method of ascent; a switchback series of inclines; a switchback railway.
• noun A railway for ascending or descending steep acclivities, in which a practicable grade is obtained by curving the track alternately backward and forward along the side of the slope. Also called switchback railway.
• noun By extension, an inclined railway in which the movement of a train or of a car is partly or wholly effected by gravity, as in the switchback railway at Mauch Chunk, Pennsylvania, and railways constructed for purposes of amusement at watering-places, fairs, and pleasure-resorts.

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

• noun A zigzag path, road or railway track; especially a railway track in which the train travels in a reverse direction at each switch
• noun A hairpin bend.
• noun dated, UK A roller coaster.
• verb of a path, etc. To zigzag.