What is "vt-line"

The VT-line or Vammelsuu–Taipale line (; ; ) was a Finnish defensive line on the Karelian Isthmus built in 1942–1944 during the Continuation War and running from Vammelsuu on the northern shore of the Gulf of Finland through Kuuterselkä and Kivennapa and along Taipaleenjoki to Taipale on the western shore of Lake Ladoga. It crossed the Saint Petersburg– Vyborg railroad at Sahakylä (now 63rd km) and the Saint Petersburg–Hiitola railroad at Kelliö (now 69th km).