\documentclass{article}
\usepackage{fontspec}
\setmainfont[Script=Devanagari]{FreeSerif}
\newfontfamily\Chandas[Script=Devanagari]{Chandas}
\begin{document}

"pra" or "tra", "प्र" "त्र"

{\Chandas "pra" or "tra", "प्र" "त्र"}


\end{document}
