An example of derail is when an obstacle forces a train to go off the tracks; derail the train.
An example of derail is when you cut funding to a project and make it impossible to continue; derail the project.
The protesting students derailed the professor's lecture.
The derail was placed deliberately so that the train would fall into the river.
The conversation derailed once James brought up politics.
A campaign derailed by lack of funds; a policy that derailed under the new administration.