How many years is the vice driver's license for diesel locomotives valid?

According to the owner's guide, the assistant driver's license of diesel locomotive is a certificate to prove that the holder has the qualification of assistant driver of diesel locomotive. Under normal circumstances, the validity period of diesel locomotive driver's license is six years, which means that the holder must pass the review or renewal within six years to maintain the validity of his qualification. During the validity period, the holder needs to meet certain conditions before passing the examination or renewal.