Dhammer: Going back to your two trains, the way to solve the problem is simply to turn the two trains around. Send the LA one west at 60 MPH and the Boston one east at 85 MPH. The two trains would disappear from the realm of mathematics exams forever...

I guess this trip is a bonanza for the Frequent Flyer. Also, here's my SWAG at the ticket scenario:

The travel department bought two round-trip tickets. At the travelers outbound date/time, there were no seats available on UA, and at his return date/time, there were no seats available on AA. HOWEVER, buying a RT EWR-LAX on AA, with a return in the distant future AND a RT LAX-EWR with a return in the distant future was significantly cheaper than two one-way fares. Would I ever want my corporate travel dept to do this??? No...because if I fly this route again, I could run into trouble with future tickets being deemed illegal. But hey...it's a guess...

I know I delved back into the airfares a bit, but only to justify why someone would do this. Sorry, but I can't get past the fact that, it today's world, this ticket would be EXPENSIVE. The scenarios I've had to do something like this involved ALL airlines being pretty well sold-out of seats, resulting in full-Y tickets.