Thanks for the interesting reply! The situation is that SJC-BWI direct shows L>4, but the individual segments have L>0 segment 1, L=0 segment 2, the classic married segment situation, so creating two segments will not yield the L fare.

Your proposed test is interesting but doesn't work out the way you predicted. If I book the earlier SJC-IAH segment 1, segment two still yields L=0. I can only conclude that the web booking engine is not quite as smart as the agent's engine, and she (inadvertently) forced a through L fare for the two segment trip, even though segment 2 showed L=0 for both her and me. Perhaps the availability function and the booking function are somewhat divorced, so that even though the availability is 0, the booking engine is more forgiving.

The moral of this experience seems to be that if you are foiled by a married segment problem, call and ask them to book and fare it anyway, and see what happens!