documentclass{article}

usepackage{tikz}

usetikzlibrary{%
   backgrounds,%
}

tikzset{
   medium/.style={line width=2.5pt},
   thin/.style={line width=0.5pt},
   every picture/.append style={medium},
   every scope/.style={medium},
   every on background layer/.style={every picture},
}

pgfdeclarelayer{background}
pgfdeclarelayer{foreground}
pgfsetlayers{background,main,foreground}

begin{document}
1 begin{tikzpicture}
   draw (0,0) -- (1,1);
   begin{scope}[on background layer]
      draw [red] (0,1) -- (1,0);
   end{scope}
   draw (0,0) -- (1,0.5);
end{tikzpicture}
quad
2 begin{tikzpicture}
   draw (0,0) -- (1,1);
   begin{pgfonlayer}{background}
      draw [red] (0,1) -- (1,0);
   end{pgfonlayer}
   draw (0,0) -- (1,0.5);
end{tikzpicture}
quad
3 begin{tikzpicture}
   draw (0,0) -- (1,1);
   begin{scope}
      begin{pgfonlayer}{background}
         begin{scope}
            draw [red] (0,1) -- (1,0);
         end{scope}
      end{pgfonlayer}
   end{scope}
   draw (0,0) -- (1,0.5);
end{tikzpicture}
quad
4 begin{tikzpicture}
   draw (0,0) -- (1,1);
   begin{pgfonlayer}{foreground}
      draw [red] (0,1) -- (1,0);
   end{pgfonlayer}
   draw (0,0) -- (1,0.5);
end{tikzpicture}
quad
5 begin{tikzpicture}
   draw (0,0) -- (1,1);
   begin{pgfonlayer}{foreground}
      begin{scope}
         draw [red] (0,1) -- (1,0);
      end{scope}
   end{pgfonlayer}
   draw (0,0) -- (1,0.5);
end{tikzpicture}
quad
6 begin{tikzpicture}
   draw (0,0) -- (1,1);
   begin{scope}
      begin{pgfonlayer}{foreground}
         begin{scope}
            draw [red] (0,1) -- (1,0);
         end{scope}
      end{pgfonlayer}
   end{scope}
   draw (0,0) -- (1,0.5);
end{tikzpicture}
end{document}