spin one's wheels

JR四国はレール上に毛虫が大量発生し、踏みつぶした普通列車が坂を上り切れず，運休したと発表しました。ここのspinning its wheelsはどのような意味でしょうか。ninetailsくん，どうぞ。（KAwada）

Squished caterpillars stop Shikoku passenger train in its tracks

TAKAMATSU -- A Shikoku Railway Co. (JR Shikoku) train found itself spinning its wheels on May 3 after squishing so many caterpillars it couldn't make it up a low hill.
According to JR Shikoku, the train ran over a major outbreak of hairy caterpillars between Sanukiaioi and Awaomiya stations on the Kotoku Line, making its wheels so slick it couldn't get up a nearby hill, forcing it out of service. The company said the hill is not very steep.

http://mainichi.jp/english/english/newsselect/news/20150504p2a00m0na008000c.html