\setupoutput[pdftex]

\setMPtext{testtext} {}

\startusableMPgraphic{mptext}
  draw textext.rt(\MPstring{testtext}) ;
\stopusableMPgraphic



\starttext

\setupunderbar[rulethickness=2pt]

\underbar{Thick line}

\setMPtext{testtext}{\underbar{Thick line}}

\useMPgraphic{mptext}

\setupunderbar[rulethickness=.15pt]

\underbar{Thin line}

\setMPtext{testtext}{\underbar{Thin line}}

\useMPgraphic{mptext}


\stoptext