1
general::
Suddenly switch from one thought or activity to another. For example, Joe was describing his trip to Australia and, jumping the track, began complaining about the airline, or They couldn't decide on the next step and now the whole reorganization plan has jumped the track. This expression alludes to a train going off the rails. [Colloquial; early 1900s]
American Heritage Idioms
2
general::
Phrase(s): jump the track
1. Lit. [for something] to fall or jump off the rails or guides. (Usually said about a train.) • The train jumped the track, causing many injuries to the passengers. • The engine jumped the track, but the other cars stayed on. 2. Fig. to change suddenly from one thing, thought, plan, or activity to another. • The entire project jumped the track, and we finally had to give up. • John’s mind jumped the track while he was in the play, and he forgot his lines.
McGrawhill's American Idioms And Phrasal Verbs