approximately 97% ( 97 % ) of capital lease payments relate to locomotives . rent expense for operating leases with terms exceeding one month was $ 397 million in 2018 , $ 480 million in 2017 , and $ 535 million in 2016 . when cash rental payments are not made on a straight-line basis , we recognize variable rental expense on a straight-line basis over the lease term . contingent rentals and sub-rentals are not significant . 18