Alternative to double distance in postaction for non-opacity?

Question

I was inspired by the OP in this thread: Tikz: Drawing railway tracks to make a tikzstyle that will allow me to draw arbitrarily shaped pairs of silver lines with black borders (which will actually be part of train tracks, but the examples in the linked threads do not meet my requirements). However, as stated in the comment on the linked thread, the double distance makes the area inbetween the lines opaque. Adding double=none only makes them black instead of white... but I need them to be 100% transparent. Is there a way to change my code so it works just like it does now, but the space inbetween the lines is transparent?

This is my minimal code example:

\documentclass[tikz]{standalone}
\usetikzlibrary{decorations.pathmorphing}
\usepackage{pagecolor}


\definecolor{railColor}{RGB}{200,200,200}
\newcommand{\connectionRailThickness}{0.051cm}
\newcommand{\connectionRailLineThickness}{0.013cm}
\newcommand{\connectionRailDistance}{0.367cm}


\tikzstyle{track}=[
postaction={draw=black,decorate,fill=none,double distance=\connectionRailDistance+\connectionRailThickness-\connectionRailLineThickness,line width=\connectionRailLineThickness},
postaction={draw=railColor,decorate,fill=none,double distance=\connectionRailDistance-\connectionRailThickness-\connectionRailLineThickness,line width=\connectionRailThickness},
postaction={draw=black,decorate,fill=none,double distance=\connectionRailDistance-\connectionRailThickness-\connectionRailLineThickness,line width=\connectionRailLineThickness},
]


\begin{document}
\pagecolor{yellow}

\begin{tikzpicture}
\draw[track] (0,0) to (10,0);
\end{tikzpicture}

\end{document}

Answer 1

This is not exactly what you asked for, but I think it is pretty close. The only downside is that you need to use two, rather than one, \draw lines per path. The first uses outlined track and the second track. Both styles use a slightly modified version of Mark Wibrow's answer.

\begin{tikzpicture}
  \draw[outlined track] (0,0) to (10,0);
  \draw[track] (0,0) to (10,0);
  \begin{scope}[shift=(-45:4)]
    \draw [outlined track] (-2,5) to (0,5) to[out=  0,in=270] (2,8);
    \draw [track] (-2,5) to (0,5) to[out=  0,in=270] (2,8);
    \draw [outlined track]  (6,5) to (4,5) to[out=180,in=270] (2,8) to (2,10);
    \draw [track] (6,5) to (4,5) to[out=180,in=270] (2,8) to (2,10);
  \end{scope}
\end{tikzpicture}  

I think the result is not bad in terms of the output:

Ideally, a little more would be automated, but the basic idea seems fairly sound. Essentially, we first draw a slightly enlarged version of the tracks in the outlining colour (black by default) and then draw a normally-sized version in the filling colour (gray by default). Done this way, we can also include the sleepers by adjusting the dash pattern definition used in the tracks decoration's code. First, we adjust the on-off pattern to allow for the different sizes of the thicker outlining lines and the thinner filling lines. Then we use dash phase to offset it appropriately in similar fashion.

This is all very trivial. The code is essentially Mark's - I've just added a few more keys, another style and a very small amount of flexibility to the part of the decoration responsible for the sleepers.

\documentclass[border=10pt]{standalone}
\usepackage{tikz}
\usepackage{pagecolor}

% addaswyd o ateb Mark Wibrow: https://tex.stackexchange.com/a/198114/
\usetikzlibrary{decorations,fit,fadings}

% Layers
\pgfdeclarelayer{sleeper}
\pgfdeclarelayer{rail}
\pgfsetlayers{sleeper,rail,main}

\pgfdeclaredecoration{tracks}{final}{%
  \state{final}{%
    \pgftransformreset% <- I think is possibly vital.% <- cfr: why?
    %
    % Get bounding box of decorated path as a node.
    % Must do it this way using basic layer.
    \pgftransformshift{\pgfpointanchor{current path bounding box}{south west}}%
    \pgfcoordinate{@1}\pgfpointorigin
    \pgftransformshift{\pgfpointanchor{current path bounding box}{north east}}%
    \pgfcoordinate{@2}\pgfpointorigin
    \node [fit=(@1)(@2), inner sep=\railsep+2*\railwidth] (@@) {};
    %
    % Create a fading for the track.
    \pgfinterruptpicture%
    \begin{tikzfadingfrompicture}[name=tracks]
      \path[draw=transparent!0, line width=\railsep+2*\railwidth, postaction={draw=transparent!100, line width=\railsep}] \pgfextra{\pgfsetpath\pgfdecoratedpath};
      \useasboundingbox (@@.south west) (@@.north east);
    \end{tikzfadingfrompicture}%
    \endpgfinterruptpicture
    %
    % Draw sleepers.
    \ifx\sleeperlayer\emptylayer\else\pgfonlayer{\sleeperlayer}\fi
    \draw [draw=\sleepercolor, line width=\sleeperlength, dash pattern=on \sleeperwidth off {\sleepersep-\sleeperwidth+\sleeperowidth}, dash phase={0.5*(\sleeperwidth-\sleeperowidth)}, every sleeper/.try] \pgfextra{\pgfsetpath\pgfdecoratedpath};
    \ifx\sleeperlayer\emptylayer\else\endpgfonlayer\fi
    %
    % Draw the track
    \ifx\raillayer\emptylayer\else\pgfonlayer{\raillayer}\fi
    \fill [path fading=tracks, fit fading=false, fading transform={shift=(@@.center)}, fill=\railcolor] (@@.south west) rectangle (@@.north east);
    \ifx\raillayer\emptylayer\else\endpgfonlayer\fi
  }
}
\def\emptylayer{}
\tikzset{%
  track/.style={
    decoration=tracks, decorate
  },
  outlined track/.style={
    track,
    decorations/.cd,
    rail width=\railowidth,
    rail sep=\railisep,
    rail color=\raildraw,
    sleeper color=\sleeperdraw,
    sleeper width=\sleeperowidth,
  },
  decorations/.cd,
  rail sep/.store in=\railsep,
  rail inner sep/.store in=\railisep,
  rail width/.store in=\railwidth,
  rail inner width/.forward to=/tikz/decorations/rail width,
  rail outer width/.store in=\railowidth,
  rail color/.store in=\railcolor,
  rail draw/.store in=\raildraw,
  rail layer/.store in=\raillayer,
  sleeper sep/.store in=\sleepersep,
  sleeper width/.store in=\sleeperwidth,
  sleeper inner width/.store in=\sleeperiwidth,
  sleeper inner width/.forward to=/tikz/decorations/sleeper width,
  sleeper outer width/.store in=\sleeperowidth,
  sleeper length/.store in=\sleeperlength,
  sleeper color/.store in=\sleepercolor,
  sleeper draw/.store in=\sleeperdraw,
  sleeper layer/.store in=\sleeperlayer,
  rail sep=4pt,
  rail inner sep=3pt,
  rail inner width=1pt,
  rail outer width=2pt,
  rail color=gray,
  rail draw=black,
  rail layer=rail,
  sleeper sep=6pt,
  sleeper inner width=1pt,
  sleeper outer width=2pt,
  sleeper length=10pt,
  sleeper color=gray,
  sleeper draw=black,
  sleeper layer=sleeper,
}

\begin{document}
\pagecolor{yellow}
\begin{tikzpicture}
  \draw[outlined track] (0,0) to (10,0);
  \draw[track] (0,0) to (10,0);
  \begin{scope}[shift=(-45:4)]
    \draw [outlined track] (-2,5) to (0,5) to[out=  0,in=270] (2,8);
    \draw [track] (-2,5) to (0,5) to[out=  0,in=270] (2,8);
    \draw [outlined track]  (6,5) to (4,5) to[out=180,in=270] (2,8) to (2,10);
    \draw [track] (6,5) to (4,5) to[out=180,in=270] (2,8) to (2,10);
  \end{scope}
\end{tikzpicture}  
\end{document}