I want to generate vertical lines between columns of a table. I am using FO to generate a pdf.
How can I do this?

The usual approach is to set a border for the
table cells, for example on the cells making
up the left column:
  <fo:table-cell border-right-width="1pt"
    border-right-style="solid"> ...
If this doesn't work because your cells have
a margin, try to put the cell content in a
separate block and define the margin there
instead on the cells.