How to tell csquotes to italicize quotes?

You could change the french style. But be aware that \enquote could e.g. be used in the bibliography.

\documentclass[12pt, a4paper, oneside]{book}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{lmodern}
\usepackage[french]{babel}
\usepackage[autostyle]{csquotes}
\DeclareQuoteStyle[quotes]{french}
  {\itshape\mkfrenchopenquote{\guillemotleft}}
  {\mkfrenchclosequote{\guillemotright}}
  {\itshape\textquotedblleft}
  {\textquotedblright}
\begin{document}

I like trains, so does my mom. She said : \enquote{I like trains !} clclc

\end{document}

Does this suit you?

\documentclass[12pt, a4paper, oneside]{book}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{lmodern}
\usepackage[french]{babel}
\usepackage[autostyle]{csquotes}
% erased not relevant packages
\newcommand*{\myenquote}[1]{\enquote{{\itshape#1}}}
\begin{document}
   I like trains, so does my mom. She said: \myenquote{I like trains !}
\end{document}

And just replace all enquote occurances with myenquote in your doc (be careful not to change the first one at the preamble!).

Try this:

\documentclass[12pt, a4paper, oneside]{book}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{lmodern}
\usepackage[french]{babel}
\usepackage[autostyle]{csquotes}
% erased not relevant packages
\let\oldenquote\enquote
\renewcommand{\enquote}[1]{{\itshape\oldenquote{#1}}}
\begin{document}
   I like trains, so does my mom. She said: \enquote{I like trains !}
\end{document}