lib/texinputs/railsetup.sty

 \rail@ex=\rail@x
 }
 
 % default setup for Isabelle
 \newenvironment{railoutput}%
-{\begin{list}{}{\rail@param}\def\rail@expand{\relax}\makeatletter}{\end{list}}
+{\begin{list}{}{\rail@param}\def\rail@expand{\relax}\makeatletter}{\makeatother\end{list}}
 
 \def\rail@termfont{\isabellestyle{tt}}
 \def\rail@nontfont{\isabellestyle{it}}
 \def\rail@namefont{\isabellestyle{it}}