New Line character \\ not respected in TikZ

Question

The new line character \\ doesn't seem to work in TikZ which is strange and I am not able to understand why.

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{positioning}
\usetikzlibrary{arrows}
\usetikzlibrary{backgrounds}
\usetikzlibrary{fit}
\tikzset{squarenode/.style = {
    shape  = rectangle,
    fill   = gray!50,
    draw   = black,
    thick,
    minimum height = 1cm,
    minimum width  = 1cm
}}

\tikzset{circlenode/.style = {
    shape  = circle,
    fill   = blue!20,
    draw   = blue,
    thick,
    minimum size = 1cm
}}

\tikzset{help lines/.style=very thin}
\tikzset{My Grid/.style={help lines,color=blue!50}}

\begin{document}
\begin{tikzpicture}
  [>=stealth']
  %\draw[My Grid] (-5,-5) grid (15,15);
  \node (r1) at (-4,4) [squarenode] {r1};
  \node[right=4cm of r1] (r2) [squarenode] {r2};
  \node[below=3cm of r1] (r3) [squarenode] {r3};
  \node[right=4cm of r3] (r4) [squarenode] {r4};
  \node[right=1.5cm of r1] (s2) [circlenode] {s2};
  \node[above=1cm of s2] (s1) [circlenode] {}; %s1
  \node[below=1cm of s2] (s3) [circlenode] {s3}; %s3
  \node[above=1ex of s3,red] {$s \leq 3$};
  \node[right=1.5cm of r3] (s4) [circlenode] {s4};
  \node[below=1cm of s4] (s5) [circlenode] {}; %s5
  \draw[fill=black] (s1.center) circle [radius=0.1cm];
  \draw[fill=black] (s5.center) circle [radius=0.1cm];
  \draw[->] (r1) -- (s2);
  \draw[->] (s2) -- (r2);
  \draw[->] (r3) -- (s4);
  \draw[->] (s4) -- (r4);
  \draw[->] (s1) to [out=180,in=90] (r1);
  \draw[->] (r1) to [out=-90,in=180] (s3);
  \draw[->] (s3) to [out=0,in=-90] (r2);
  \draw[->] (r2) to [bend right=45] node[auto,swap] {2} (s1);
  \draw[->] (r3) to [out=90,in=180] (s3);
  \draw[->] (s3) to [out=0,in=90] (r4);
  \draw[->] (r4) to [bend left=45] node[auto] {2} (s5);
  \draw[->] (s5) to [bend left=45] (r3);
  \begin{scope}[on background layer]
    \node [fill=gray!15,rounded corners,fit=(s1) (s3) (s5) (r1) (r2)] (R1) {};
  \end{scope}
  \node[right=2ex of R1,midway] {replacement of\\ 
  the \textcolor{red}{capacity}\\ 
  by \textcolor{red}{two places}};
\end{tikzpicture}
\begin{tikzpicture}[xshift=2cm]
  \node (r1) at (-4,4) [squarenode] {r1};
  \node[right=4cm of r1] (r2) [squarenode] {r2};
  \node[below=3cm of r1] (r3) [squarenode] {r3};
  \node[right=4cm of r3] (r4) [squarenode] {r4};
  \node[right=1.5cm of r1] (s2) [circlenode] {s2};
  \node[above=1cm of s2] (s1) [circlenode] {}; %s1
  \node[below=1cm of s2] (s3) [circlenode] {s3}; %s3
  \node[above=1ex of s3,red] {$s \leq 3$};
  \node[right=1.5cm of r3] (s4) [circlenode] {s4};
  \node[below=1cm of s4] (s5) [circlenode] {}; %s5
  \draw[fill=black] (s1.center) circle [radius=0.1cm];
  \draw[fill=black] (s5.center) circle [radius=0.1cm];
  \draw[->] (r1) -- (s2);
  \draw[->] (s2) -- (r2);
  \draw[->] (r3) -- (s4);
  \draw[->] (s4) -- (r4);
  \draw[->] (s1) to [out=180,in=90] (r1);
  \draw[->] (r1) to [out=-90,in=180] (s3);
  \draw[->] (s3) to [out=0,in=-90] (r2);
  \draw[->] (r2) to [bend right=45] node[auto,swap] {2} (s1);
  \draw[->] (r3) to [out=90,in=180] (s3);
  \draw[->] (s3) to [out=0,in=90] (r4);
  \draw[->] (r4) to [bend left=45] node[auto] {2} (s5);
  \draw[->] (s5) to [bend left=45] (r3);
  \begin{scope}[on background layer]
    \node [fill=gray!15,rounded corners,fit=(s1) (s3) (s5) (r1) (r2)] {};
  \end{scope}
\end{tikzpicture}
\end{document}