"ＪＲ江差線の線路に針金の束、信号切り替わらず" means that "There was a bunch of wires on a railway of JR Esashi-Line, so the signal didn't switch to the other". "ず" of "切り替わらず" means "not", and this is used in proverbs and written languages.

切り替わらず is consist of 切り替わる + ず. "verbs + ず" is "not + verbs" in english. ず is old or formal form of "ない". in Japanese grammar, verbs change their form depends on auxiliary-verbs. Japanese verbs end in "-u" like 切り替わる (kirikawar-u). it's dictionary form. when you use ず or ない, the "-u" turns into "-a" like 切り替わら (kirikawar-a). it's called imperfective form.

Thank 김영 Mao and Ryoko! Could you explain me grammar point of ~らず. The purpose of changing form from "切り替わる" to "切り替わらず" why do they have to change the verb like this?

I agree with Mao-san's answer.

'切り替わらず' I asked my friend who is also learning Japanese and he told me it means 'Does not toggles/switched'. But I don't know.