For instance: there was a woman who was murdered around the 1950s, in which she was found dead on the train tracks but the case remains unsolved. At the time: DNA was not even a thing nor was forensics even available, and there wasn't any CCTV during that era or "people using an iPhone to record everything" then, how do you solve this type of case with modern tools today?

It also depends on the age the suspect was back then will factor whether they're alive or dead if caught today, if they were young adults (like 25-34) then presumably they'll be around 80+ in the present. Even with genealogy, is it worth going after them if they're alive? But, if they are already dead, then both DNA & forensics are considered useless as the suspect is gone (they died).