steel safety wheel
/ˈstiːl ˈseɪfti ˈwiːl/
n
English · steel safety wheel中文 · 钢制安全轮Español · rueda metálica de seguridadFrançais · roue de sécurité
意味：Auxiliary regular train wheel that comes in contact with the running rail in the event the tire deflates and during switching.