narrow gauge

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

• noun A distance between the rails of a railroad track that is less than the standard width of 56.5 inches (143.5 centimeters).
• noun A locomotive, car, or railway line of this gauge.

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

• noun A railway gauge (distance between the two lines) that is less than the standard gauge (often quoted as 56 inches)

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

• noun a railroad track (or its width) narrower than the standard 56.5 inches