Hisatsu Line was a part of Kagoshima Main Line until seaside line of Kagoshima Main Line opened. After that the line was demoted to a local line. It passed through spectacular mountainous area of southern Kyuusyuu and had famous Okoba great loop. The grade of the line was quite steep as 30.3 permil. I spent the longest time on Hisatsu Line in my railway photography. The locos used were D51 class heavily equipped for harsh condition of the line. They were equipped with “smoke diverter” and heavy fuel oil tank. The D51 class was the most popular freight loco in JNR, but heavily equipped D51's on Hisatsu Line had quite different impression from standard ones. I preferred these modified D51's to ordinary ones.
JNR D51 Class
D51 class was the most produced JNR locos, 1,115 in total and modernised version of D50 class. The modern JNR steam loco design practice was well established with this D51 class and C57 class and all later models followed the same practice as them. The earlier 95 locos had long one piece cover from the stack to dome including water heater and sand box inside and called “slug.” After finishing first loco it was revealed tail-heavy because the designer miss-calculated weight distribution. To overcome this, the water heater was moved in front of the stack from next batch.
I have no idea how this device is called in English and never saw it outside Japan. This is an equipment to protect engineers in cabin from the smoke while running in tunnels.
In normal operation the shutter of the smoke diverter is open and smoke comes out directly from the stack. If the loco enters the tunnel the smoke hits the ceiling of the tunnel and reflects down and surrounds the cab. However if the shutter of the smoke diverter closed, the smoke pushed backward almost horizontally in high speed and passes the top of the cab.
This was a common equipment installed on JNR locos operated on mountainous lines with lot of tunnels.