QUICK ANSWER
Use Skyway after security. It is the above-ground automated train between terminals.
01
Subway
Use the underground Subway before security; it also serves the Airport Marriott.
02
Frequency
Houston Airports says Skyway departs about every two minutes and Subway about every three minutes.
03
Do not exit security by mistake
Connecting passengers already airside should not leave security just to use the Subway.
Frequently asked questions
Which IAH train is airside?
Skyway.
Which IAH train serves the Marriott?
The landside Subway.
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Sources and verification
Time-sensitive airport, airline, visa, security and transport details should always be confirmed with the relevant official source before travel.