Usually rail trails are deliberately constructed and maintained however. Although the degree to which this is done varies. And it's also not uncommon for some section of a rail trail to have been worked on and other sections are in the plans for someday--whether because of money or local opposition.

ADDED: And as noted elsewhere the railroad or its successors in ownership often still own the right-of-way--however unlikely it would be to revert a popular rail trail to rail use.