Working in cab on D9N, and started down out of the cab. Stepped down on track and twisted left knee. Heard it pop, but thought it would be ok. Did not feel need to go to doctor. On 8/15/14 he was examined and scheduled for MRI, but returned to work. He missed work 8/19/14 due to pain but returned on 8/20/14 and worked through 8/26/14. MRI scheduled for 9/2/14.