Iowa Code § 384.64: Assessment to railway company.

The right-of-way of a railway company is subject to special assessments for public improvements, and such assessments constitute a debt due the city which is a paramount lien upon the track of the railway company owning or leasing the right-of-way within the limits of the city. The property of a railway to which a lien for unpaid special assessment has attached may not be released from the lien until the whole assessment is paid.