The effect of population density on high speed rail

What is the effect of population density on high speed rail?
The economic research paper should be 2750 words long and consists of Introduction, Literature Review, Data, Theory, Estimation, and Results. OLS Multiple Regression should be conducted in this paper. However, an economic model is not needed in this paper because my professor said an economic model sometimes does not fit in a research paper and it possibly makes the research paper dirty. The main variable should be ‘population density’ and you need to set up some control variables.