Two ways of looking at it:
1. Cross border traffic plummets as people stay their own side and the tourist trail from the south to the north evaporates.
2. With long queues of lorries at a reinstated border, passengers flock to the train.
I wonder.
Then there's NIR; who will fund it? There will be no more EU grants. With Cameron in no mood to fund any of his "regions" any more, if he gives the north more money to make up what it loses in EU funds, Scotland and wales will also want their slice, as will England, so he's not going to be able to do that, as sterling will almost certainly devalue anyway!
Will EU-wide railway operating regulations be followed by NIR any more? If not, does that mean that IE and NIR will have different sets of rules after a while? If so, will NIR and IE crews have to have totally separate PTS passes?