Admit it: When chirpy holiday melodies permeate your earbuds and you see Santa and his reindeer cruising toward you on a decked-out CTA train, you become just as giddy as the toddler standing next to you. What began as a small-scale operation 30 years ago — when the CTA modestly decorated an out-of-service train car and used it to deliver holiday meals to charitable organizations — has grown into the Yuletide bonanza that tens of thousands of us pack the rails to experience each season. With candy-cane-dealing elves aboard and 100,000 twinkling lights per train, it’s enough to make even the Grinchiest commuter forgive the L for its imperfections.