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2.
A train of thought is a process linking one thought to another giving a common thread.
A train of thought is used by people in debating societies (eg the Oxford Union, a famous Debating Society). Sometimes it is hard to follow a train of thought as some people tend to ramble on.
by St. Ias November 02, 2005
 
1.
Similar to the subway of unconsciousness and the airplane of logic, the train of thought is used to casually describe one's thought process. It is usually lost, sometimes derailed, frequently hijacked, and has been known to reverse course. Also, the train of thought is never stated to have a beginning or an end, and it is unknown whether there are any passengers on board.
Dang it, I lost my train of thought again.
by dan.s July 21, 2008