The reason you are not seeing the black line is because although you have specified a width of 1px the size of the cell has been reduced to 0px. I know that table cells ignore the width and expand if they contain content that cannot be wrapped. It probably tried to readjust the cell widths to try and get it within the screen’s viewport.
Might an alternative approach be to put a border-right or border-left style with a value of 1px solid black.