Crashing is adding more resources to the project to reduce the project schedule. Fast tracking is finding more work to do in parallel (without adding resources) to reduce the project schedule. So they are different techniques with the same goal -- reducing the time it takes to complete a project.
Both need to be approached carefully. In complex projects, there may be time needed for resources to come up to speed if they are added in the middle of a project. Likewise doing more work in parallel may introduce more risks.