Bar line thickness

Adjusting LilyPond bar line thickness does nothing.

   \new Staff {
      c'1
      c'1
      c'1
      \override Staff.BarLine #'color = #red
      \override Staff.BarLine #'thickness = #0.5
      c'1
   }

Adjusting bar line hair-thickness does the trick.

   \new Staff {
      c'1
      c'1
      c'1
      \override Staff.BarLine #'color = #red
      \override Staff.BarLine #'hair-thickness = #0.5
      c'1
   }

Using LilyPond dummy bars

Line- and page-breaks require bars. This is true even for midmeasure breaks which require the dummy construction \bar "". Midmeasure breaks also necessitate the removal of the Forbid_line_break_engraver from all Voice contexts.

   \layout {
      \context { \Voice \remove Forbid_line_break_engraver }
   }

   \new Score <<
      \new Staff <<
         \new Voice { s1 s2 \bar "" \break s2 s1 }
         \new Voice { c'1 c'1 c'1 }
      >>
      \new Staff { c'1 c'1 c'1 }
      \new Staff { c'1 c'1 c'1 }
      \new Staff { c'1 c'1 c'1 }
   >>

Fortunately, each midmeasure break requires only one such dummy bar. The midmeasure break here cuts across four different vertically stacked Voices.