The parallel to the airport check-in procedure seems valid. Aircraft carriers carry some responsibility for the passengers. If they don't have proper documents they might be refused entry at the destination and returned at the expense of the airline. As personnel carriers the railways seem currently to face similar demands from the authorities on the border between Denmark and Sweden.
The threat to airlines and other transport companies does not only consist in having to organize and pay the travel back to destination but also heavy fines independently of wheteher the Company resosibles are guilty or not in any way or condition.
Best regards, ULF