The Roosevelt_Boulevard_Subway idea has been floated around since 1913. Unless there is adequate funding & a genuine interest in getting such a projects done, they just remain in the planning stage.
https://en.wikipedia.org/wiki/Broad_Street_Line#Roosevelt_Boulevard
"The Roosevelt Boulevard subway line.
“In the Northeast, it’s like a quasi-urban legend,” Fitzpatrick, now 30 and living in Fishtown. “Like, ‘Oh yeah, the Boulevard subway.’ Some people would talk about it with optimism, some people would say, ‘can you believe they thought that?’” https://billypenn.com/2022/08/12/roosevelt-boulevard-subway-northeast-philadelphia-public-transit-infrastructure-funding/
https://therabbitportal.blogspot.com/search?q=The+Roosevelt+Boulevard+Subway
