There would be two major impacts of such a terrorist attack.
First, there would be a tremendous economic impact. A significant percentage of goods that are shipped from one place to another in the United States spend at least part of their journey on trains. If all trains stopped for any appreciable amount of time, the economy would be hamstrung. For example, many electric generating plants run on coal that is brought to them by rail. These plants might eventually stop running, thus depriving the US economy of much needed power. Many other impacts would occur. The American economy would suffer.
However, the psychological impact of such an attack might be even greater than the economic impact. Any attack on the US that was so serious that all train traffic needed to be shut down would have to be something on the level of the attacks of September 11, 2001. Another such attack would make Americans feel vulnerable again as we did in the days after those attacks. It could well lead to a loss of confidence on the part of the American people. This would have political and economic consequences.
Thus, an attack of that kind of magnitude would have a tremendous economic, political, and psychological impact on the US.