Dallas and nearby Fort Worth were initially developed due to the construction of major railroad lines through the area allowing access to cotton, cattle and later oil in North and East Texas. The construction of the Interstate Highway System reinforced Dallas’s prominence as a transportation hub, with four major interstate highways converging in the city and a fifth interstate loop around it. Dallas then developed as a strong industrial and financial center and a major inland port, due to the convergence of major railroad lines, interstate highways and the construction of Dallas/Fort Worth International Airport, one of the largest and busiest airports in the world. In addition, Dallas has DART (Dallas Area Rapid Transit) with different colored train lines that transport throughout the Metroplex.