It was already dark in the evening, but I saw a cat walking on the power line. W-wait, can cats really do that?
Literal
Evening [at-で] already dark was [explanatory-んですけど], cat [subject-が] power-line [object-を] crossing [nominalizer-の][object-を] saw. S-so-so, such-thing can-do [question-んですか], cats [topic-って].
電線を渡る 'to cross/walk along a power line.' The stuttered そそそ 'w-w-wait' conveys genuine surprise. ネコって at the end uses って as a casual topic marker, making 'cats' the subject of the amazed question. The whole passage is charmingly incredulous.