With 6610 meters, this will be the longest tunnel in the 82,5 kilometer long Dimapur - Kohima New Rail Line. The railway line will have 21 tunnels of total length 31 km, in addition to many other structures like bridges etc.

https://nagalandpage.com/construction-for-longest-tunnel-in-dimapur-kohima-new-rail-line-project-begins/