\startuseMPgraphic{verticalline}
    if CurrentColumn < NOfColumns :
        draw rightboundary OverlayBox shifted (2.5mm,0) withpen pencircle scaled .5mm withcolor \MPcolor {darkred} ;
        setbounds currentpicture to OverlayBox enlarged 2.5mm;
    fi ;
\stopuseMPgraphic

\setupalign
  [verytolerant,stretch]

\dontcomplain

\definecolumnset
  [example]
  [n=3,
   background=verticalline,
   distance=5mm]

\defineoverlay
  [verticalline]
  [\useMPgraphic{verticalline}]

\starttext

    \startcolumnset [example]
        \input knuth
        \input knuth
        \input knuth
        \input knuth
    \stopcolumnset

\stoptext
