I don't know about the longest, but the trains that run in the CIC section between SGRL and DHN cover the distance from Chopan to Billi Jn twice, but that's only 6 km.

CIC section ke beech chalne wale train SGRL aur DHN ke beech Chopan se Billi Jn tak do baar jaate hai, lekin sirf 6 kilometer hai ye to. Sabse lamba kaun hai, mujhe pata nahi.

Maybe this one; any train for North East that goes back and forth from Durgapur to Andal, which is about 17 km twice.

Besides, the Ranchi Ajmer train used to have a similar situation where it goes to BSB from DDU, then changes direction at BSB and continues to MZP, overlapping till Block Hut B Cabin for around 15 km. But now the train is cancelled.

Also, I think the Mussoorie Express has a similar reversal of about 12 km near Muzzampur Narayan Junction.

Shayad yeh wala; koi bhi North East ki train hai jo Durgapur mein reversal karti hai aur forth and back DGR se Andal tak do baar 17 km ghoomti hai. Uske alawa pehle Ranchi Ajmer ko bhi same fate mili thi, DDU se BSB jati hai aur BSB mein reversal ke baad MZP ke liye alag hokar jati hai aur dono trains block Hut b cabin tak around 15 km mein overlap karti hai. Lekin abhi train cancelled hai. Main sochta hun ki mussoorie express ki taraf bhi around 12 km ke pass reversal hota hai, muzzampur Narayan jn ke paas.

That train really seems to dig Lucknow, man. It stops at like five stations here and hangs out for a whopping two hours. And get this, the AC coaches are a total waste from Lucknow to Mailani 'cause the locals prefer the General/Sleeper coaches to go to Sitapur/Mailani. I hopped on a few AC coaches and there were like only a few people squished in there. So, I'm thinking if I book a seat in this train from Lucknow to Mailani in the SL class, there's a good chance I'll get upgraded, bro.

Voh train itna Lucknow pasand karti hai ki yahan paanch stations pe ruk jaati hai aur do ghante tak wahan bitaati hai. Aur uski AC coaches toh Lucknow se Mailani tak bekar hai kyunki log Sitapur ya Mailani jana ke liye local train me jaate hai aur General/Sleeper coaches me safar karte hai. Maine kuch AC coaches me jaake enter kiya tha aur ek digit ke passengers the. Lagta hai agar main is train me SL class me Lucknow se Mailani ki seat book karu toh upgrade hone ki achhi sambhavna hai.

Mussoorie really goes twice over the muazzampur Narayan-najibabad stretch, which is 11.2 km long. Meanwhile, the Nagaon and Ntsk Tambaram Express go twice over the Durgapur andal stretch, which is 16.9 km long, I think that's the longest one like that. A lot of trains coming from Merta and going to Phalodi go twice over the bgkt-Jodhpur stretch, which is only 2 km long. Trains coming from Ckp to Tata, reversing up, and then heading towards Chandil go twice over the gamharia Tata stretch, which is 11 km long.

Mussoorie muazzampur Narayan- najibabad ke section ko do baar cover karta hai (11.2 kilometer),
jabki Nagaon aur Ntsk Tambaram Exp Durgapur andal ke section ko do baar cover karte hai (16.9 kilometer), shayad sabse lamba.
Bahut train merta se Phalodi jaane wali bgkt-Jodhpur ke section ko do baar cover karti hai (2 kilometer).
Ckp se tata jaane wali train gamharia tata ke section ko do baar cover karti hai (11 kilometer), upar reverse karke chandil ki taraf jaate hue.

This Kurj CNB Passenger is like a boss, covering the Banda Khairar section of about 10 kilometers not once, but twice.

Kurj cnb passenger banda khairar ke lagbhag (10kms) tak do baar chalti hai.