Sir single line me single dono side me hota hain. Toh jab doubling hai kabhi kabhi railways dusra patri jo banti hai usme vi dono side me signal bana deti hain. jisse fully electrified double line nahi hota hai balki fully electrified double single line ho jata hain. issme dono side me traffic chalaya ja sakta hain.
Example :- Dhanbad Chandrapura Line, Andal Sainthia Line.
DC line me koi fixed up/down line nahi hain dono side traffic ja sakti hain.