[Tex/LaTex] Drop brackets around reference item in `natbib`

bibliographiesbibtexnatbib

I am writing for a journal (Transportation Research Record) that has strange and particular bibliographic guidelines. I've been able to adapt natbib to work for the in-text citations. But the bibliography prints brackets around the item numbers. 

\documentclass[11pt]{amsart}
\usepackage{graphicx}
\usepackage{amssymb}
\title{Brief Article}
\author{The Author}
    \usepackage[round,sort,numbers]{natbib}
    \newcommand{\trbcite}[1]{({\it\citenum{#1}})}
\begin{document}
\maketitle

This is an example citation \trbcite{Mokhtarian2008}.
\bibliographystyle{plain}
\bibliography{bibfile}
\end{document}

enter image description here

With bibfile.bib containing

@article{Mokhtarian2008,
author = {Mokhtarian, Patricia L. and Cao, Xinyu},
doi = {10.1016/j.trb.2007.07.006},
issn = {01912615},
journal = {Transportation Research Part B: Methodological},
keywords = {built environment,causality,land use,residential location,smart growth},
month = mar,
pages = {204--228},
title = {{Examining the impacts of residential self-selection on travel behavior: A focus on methodologies}},
url = {http://linkinghub.elsevier.com/retrieve/pii/S0191261507000744},
volume = {42},
year = {2008}
}

Any easy ways to do this? Or is this something that will require the .bst file?

Best Answer

Add to the preamble

\makeatletter 
\renewcommand\@biblabel[1]{#1} 
\makeatother

Not related to the question, but \it is an old command that shouldn't be used anymore; use \itshape instead.

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