Koznitsa is a railway tunnel under the Koznitsa ridge in the Stara planina

Balkan Mountains

The Balkan mountain range is a mountain range in the eastern part of the Balkan Peninsula. The Balkan range runs 560 km from the Vrashka Chuka Peak on the border between Bulgaria and eastern Serbia eastward through central Bulgaria to Cape Emine on the Black Sea...

 (Balkan Mountains), the longest tunnel in Bulgaria

Bulgaria

Bulgaria , officially the Republic of Bulgaria , is a parliamentary democracy within a unitary constitutional republic in Southeast Europe. The country borders Romania to the north, Serbia and Macedonia to the west, Greece and Turkey to the south, as well as the Black Sea to the east...

. The tunnel is located between the Bulgarian towns of Koprivshtitsa

Koprivshtitsa

Koprivshtitsa is a historic town in Sofia Province, central Bulgaria, lying on the Topolnitsa River among the Sredna Gora mountains. It was one of the centres of the April Uprising in 1876 and is known for its authentic Bulgarian architecture and for its folk music festivals, making it a very...

 and Klisura

Klisura

Klisura is a small town in the Karlovo municipality of the Plovdiv Province in central Bulgaria. It is situated in a valley surrounded by the Balkan Mountain range and Sredna Gora...

 and is 5,812 m long. It is the seventh tunnel on the Sofia

Sofia

Sofia is the capital and largest city of Bulgaria and the 12th largest city in the European Union with a population of 1.27 million people. It is located in western Bulgaria, at the foot of Mount Vitosha and approximately at the centre of the Balkan Peninsula.Prehistoric settlements were excavated...

–Karlovo

Karlovo

Karlovo is a picturesque and a historically important town in central Bulgaria located in a fertile valley along the river Stryama at the southern foot of the Balkan Mountains...

line, starting at the exit of Koprivshtitsa station and ending just before Strema station. Along its entire length the tunnel is straight and at Koprivshtitsa the lights of the locomotive are visible a long distance into the tunnel.