To sign subway, hold your non-dominant forearm horizontally in front of your body, your hand flat and facing down. Then form your dominant hand into the ASL letter Y sign, palm side facing up, and starting from underneath your non-dominant palm, slide your upturned 'Y' hand back and forth, from underneath your wrist to your finger tips, then back again.
Your 'Y' hand is mimicking the movement of trains in the tunnel of the subway.
Teach your toddler the subway sign when you are taking them on their first time to hop on a city subway train.
Teach this sign along with the other flash cards pertaining to the many ways you can travel in the city.