David Overton was told by TV judges his washable, wearable maps were "too niche" and "would look ridiculous".
But a chance meeting with fellow passenger and Lush founder Mark Constantine led to an order for 1,800 SplashMaps.
A third have already been sold in stores as scarves and gift-wrap.
Dragon Sarah Willingham told Mr Overton she would "look ridiculous wearing the A to Z around my neck" when the show aired on Sunday.
Fellow judge and financier Peter Jones also predicted a bleak future and said: "I don't see how you can make a lot of money".
But prior to filming in April, Mr Overton said he struck up a brief conversation with Mr Constantine on a train, not knowing he was the head of a major high street firm.
He said it started because he was wearing one of his creations as a scarf.
Cosmetics retailer Lush, based in Poole, Dorset, contacted Mr Overton about a week after his ordeal with the Dragons and placed an order.
Mr Constantine said he thought the maps were a "great idea" and "hoped the collaboration would continue".
Mr Overton said: "It means a heap more than the Dragons because here's a guy who believes in the product and knows that he can make a good profit."
A failed Dragon's Den contestant has signed a deal with a big-name cosmetics brand after unknowingly meeting its founder on a train.