It does seem a bit odd though for the drivers to resign - as others have said, red light comes on, refuse to drive.  If you are then disciplined/suspended etc - force the companies hand to put in writing the company position was they should have continued driving, in breach of their own policy, now in writing.  I suspect GNE wouldn't get far in the barrage of emplument tribunals that would follow.

While I do sympathise with the drivers in the article (and wider - it does sound an awful toxic organisation to work for) I suspect this is a bit less one sided than it's being portrayed by the unions.  Surely the issue is being required to drive a dangerous vehicle - whether you've signed bit of paper or not the outcome could still be calamitous and you'd still be culpable in the eyes of the law in control (or not, as the case may be) of a defective vehicle even if you havent signed a bit of paper for GNE.