O gauge manufacturers tend to make the gap between cars larger than scale. This is due to the extra room required for clearances on O-27 (and even O-72) curves. Remember, these all started out as toys! That said, some of today's manufacturers have made provision for tightening the spacing and using K-D couplers. Speaking of which, the normal "lobster claw" couplers we are accustomed to are massively over sized. You can thank the Lionel corporation for that. I suspect they did it to allow for easier uncoupling by children.