Amazon’s Unlikely Secret to Delivering So Fast

Amazon.com’s New York City delivery route at times takes an underground detour.

The online retail behemoth has begun to utilize New York City’s subway system for the company’s ultra-fast service, known as Prime Now. Prime Now promises to deliver popular items in as little as an hour for $7.99, or within two hours for free.

Two Amazon delivery workers were observed pushing large trolleys of Amazon boxes on the subway, and they told the Financial Times that they had been using the subway trains for most Prime Now deliveries because traffic in Manhattan made it “impossible to honor a 60-minute guarantee.

Amazon later confirmed the strategy, telling FT: “In Manhattan, our folks bike, walk or use public transportation. They only drive if the item is large like a flat screen TV.”

Amazon’s speedy service debuted in parts of New York; it’s part of a growing trend by major retailers to improve their delivery times in a battle for online market share.

This article originally appeared on Fortune.com.