jump the track

jump the track (suddenly switch from one thought or activity to another) — букв. сойти с рельсов; || перескакивать с одного на другое

Example 1:
Example 2:

Example 3: Joe was describing his trip to Australia and, jumping the track, began complaining about the airline.
(answers.com)

Example 4: They couldn't decide on the next step and now the whole reorganization plan has jumped the track. (answers.com)

Related vocabulary
[go off at a tangent]
[beat about the bush]
sidetrack sb

======
This expression alludes to a train going off the rails. (The American Heritage)