Front car (closest to engine) for Terminals 1 or 3. You'll be closest to escalators, avoid crowd bottlenecks.

Middle cars for Terminal 2 — Platform is longer, you exit center.

RER B splits at Mitry-Claye. Make sure you're on train marked "Aéroport Charles de Gaulle" not "Mitry" or you'll add 30 minutes.

35-40 minutes from Châtelet-Les Halles. Trains every 10-15 minutes but check weekend engineering works on RATP website.