Double line may refer to:

• Double-track railway, a type of railway with two tracks, usually involving running a track in each direction
  • Double line automatic signalling, a form of railway signalling used on the majority of double track sections in New Zealand
• Double-lined mackerel, Grammatorcynus bilineatus, a species of Spanish mackerel
• Double-lined spectroscopic binary, a type of binary star: see Binary star#Spectroscopic binaries
• Mythimna turca, common name the double line, a moth of the family Noctuidae
• In algebraic geometry, a certain family of conics