Comment by ahazred8ta

If you have two very very long trains several light years long, passing each other in opposite directions, and each train has FTL-comm devices, and the front and back end of each train can flash signal lamps that are synchronized with each other, your train will see the other train's rear lamp flash long before the other train's front lamp. The other train's front FTL-comm is sending messages 'back in time' to reach the other train's rear FTL-comm. By sending signals from your front, to your rear, to their front, to their rear, and back to your front, the message arrives before you sent it. So we think FTL-comm can not work.