Subway iOS 7 Style Icon Set
It’s actually a paid product, but the PNG only version is available for free.
Subway iOS 7 Style Icon Set is a free Icon design resource made by Dribbble user Pixle. As far as I can tell, its license is Free for Non-commercial.
Helpful hint: Dribbble users are nice. If the license is unknown, it is usually because they forgot to mention it explicitly; not because they were being stingy. Ask nicely and they will probably allow you to use it.