Position the line at the top left corner of the artboard and copy it (command + C then command + V) position the copy of the line on the bottom right corner of the artboard. Select the Blend tool (W) and click on the first line, then click on the second line. This will make a blend between the two. With the two lines selected, double click the Blend tool icon and set the spacing to specified distance, 1px. Click OK, then go to Object > Blend > Expand to convert the blend to lines.
Set the blend options for the lines.
With the lines selected Option + Click on the artboard with the Reflect tool (O), set the axis to vertical with an angle of 90 degrees, click Copy.