The straphanger gunned down on a Midtown subway platform was a 26-year-old security guard and Yankees fan from The Bronx who cared for his ailing dad and was widely beloved.

Darrel Rodriguez, 26, was on the way home to the University Heights apartment he shared with his father when he was shot in the chest at the 53rd Street/7th Avenue station around 12:20 p.m. Saturday, police and law-enforcement sources said.

The armed thug — who wore a red tracksuit, a black vest and face mask — ran off, leaving Rodriguez dying on the northbound E train platform, according to cops.

Sources said the two men had an argument on the platform before the assailant opened fire, though it wasn’t immediately clear what the beef was about or how it started.

But neighbors on Monday described Rodriguez as a “beautiful kid” that many of them watched grow into a “respectful,” kind and mild-mannered adult.

"He was a quiet, decent young man. Didn't look for problems with nobody,” said Maddy, 46, who declined to give her last name and told The Post she had known Rodriguez and his twin brother since they were babies.

His killer “had to be a stranger,” the neighbor said, since “everybody on the block loves him."

“He was always polite, always said hello to everybody."

Police are working on trying to identify the suspect, but are handcuffed by lack of video from the platform and the fact that his face was covered, sources said.

Willie Suzana, a barber who runs a shop across the street from Rodriguez’s apartment building, said he used to pick him up at school when he was little, and more recently would see him leaving for his overnight security job.

Sources said Rodriguez manned the front desk at a Manhattan building as a security guard, though the exact location wasn’t immediately revealed.

"I was in shock. I couldn't believe it. To see a kid you have seen grow up…” Suzana said, noting that he wasn’t playing music at his shop like he usually does as he mourned Rodriguez’s loss.

Neighbors said he has two brothers, including a twin living in Texas, and lived with his elderly father, who was partially incapacitated after suffering two heart attacks and has difficulty speaking.

A heartbreaking photo shows the three siblings in happier times, with Darrel wearing a massive smile and a Yankees cap with twin Derick and brother Gabriel.

Derick Rodriguez told the New York Times that he knew something had gone wrong Saturday when a text message he sent his brother didn't go through.