The White Castle chain has added a second vegan slider option to their menu. The New Black Bean Slider joins their Veggie Slider menu item.
Previously, The Vegetarian Resource Group reported that the bun used for these sliders is vegan. See:
http://www.vrg.org/journal/vj2016issue4/2016_issue4_white_castle.php
For more info on the sliders, see:
https://www.whitecastle.com/promotions/black-bean-slider
For information on veggie options at other restaurant chains, see:
http://www.vrg.org/fastfoodinfo.php