If you miss the first train you could just take an Uber or Lyft between the two airports. By 6:30 there's a pretty good chance that they'll have cleaned up the half dozen wrecks on I-95 that are the normal result of rush hour traffic that is composed of a 50/50 mix of South Florida idiots driving 100 mph and NY/NJ blue hairs driving 45 mph so the drive time might be reasonable.