Only if you are already in connection — with an e-ticket, boarding pass or reservation code for your next flight — or part of a flight crew. The airport reserves the Aerotrén for exactly those travellers, running 05:00 to 01:00 daily between Gate 6 (T1) and Gate 4 (T2). Everyone else, including anyone dropping off or collecting a passenger at the other terminal, needs a paid bus instead.
