In message , at 12:31:41 on
Sat, 7 May 2011, Paul Corfield remarked:

[1] except at Hong Kong International Airport. The Airport Line has
premium fares so it is separately gated off from all other lines.

Madrid metro has a premium fare to the airport (but not stations earlier
or later on the same line). I assume they enforce this with exit
barriers, I forget. I do remember having to go through barriers when
changing from one line to another near the city centre.
--
Roland Perry