sidetrack

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

• intransitive verb To divert from a main issue or course.
• intransitive verb To delay or block the progress of deliberately.
• intransitive verb To switch from a main railroad track to a siding.
• intransitive verb To deviate from a main issue or course.
• intransitive verb To run into a siding.
• noun A railroad siding.

from The Century Dictionary.

• noun A short line of rails branching off by a switch from the main line of a railroad, and either returning to it or not at the further end, for use in turning out, shifting rolling-stock, etc.; a siding.
• To put upon a side-track; shift from the main line of a railroad to a subsidiary one; shunt.
• Figuratively, to divert to one side; turn aside from the proper or the practicable course.
• To pass to a side-track; come to rest on a siding.

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

• transitive verb (Railroads) To transfer to a siding from a main line of track.
• transitive verb colloq. Hence, fig., to divert or reduce to a position or condition that is relatively secondary or subordinate in activity, importance, effectiveness, or the like; to switch off; to turn aside, as from a purpose.

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

• noun rail transport A second, relatively short length of track just to the side of a railroad track, joined to the main track by switches at one or both ends, used either for unloading freight, or to allow two trains on a same track to meet (opposite directions) or pass (same direction); a railroad siding.
• noun Any auxiliary railroad track, as differentiated from a siding, that runs adjacent to the main track.
• noun mining A smaller tunnel or well drilled as an auxiliary off a main tunnel or well.
• noun An alternate train of thought, issue, topic, or activity, that is a deviation or distraction from the topic at hand or central activity, and secondary or subordinate in importance or effectiveness.
• verb To divert (a locomotive) on to a lesser used track in order to allow other trains to pass.
• verb To divert or distract (someone) from a main issue or course of action with an alternate or less relevant topic or activity; or, to use deliberate trickery or sly wordplay when talking to (a person) in order to avoid discussion of a subject.
• verb To sideline; to push aside; to divert or distract from, reducing (something) to a secondary or subordinate position.
• verb intransitive To deviate briefly from the topic at hand.

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

• verb wander from a direct or straight course
• noun a short stretch of railroad track used to store rolling stock or enable trains on the same line to pass