In the pilot episode, a middle aged obese man in standing in an underground subway and gets killed in a train accident. He is survived by his wife.
This man's heart/brain are transplanted into a young athletic good looking man (humanoid) who works for the police department and falls for the obese man's wife.
This sounds like Now and Again.
The show originally aired from September 1999 to May 2000 on CBS, but Wikipedia confirms it also aired on AXN.
The only slight mismatch from your description is that the main character "works for" (but really is sort of owned by) a secret government organization, rather than the police.
Here is the plot synopsis from Wikipedia:
Michael Wiseman (John Goodman in the pilot and flashbacks, and subsequently Eric Close) lives in suburban New York with his wife Lisa (Margaret Colin) and daughter Heather (Heather Matarazzo) and works as an executive at an insurance company. When passed over for a promotion, Michael and his friend and co-worker Roger (Gerrit Graham) go out drinking after work. While standing on the subway platform heading home that night, Michael is accidentally knocked off the platform directly into the path of an oncoming train. When he awakes, he sees Dr. Theodore Morris (Dennis Haysbert), the head of a top secret government project to artificially engineer the perfect human body. Unable to create a brain from scratch, Dr. Morris has rescued Michael's brain from his dead body and implanted it in the engineered body. The process allows Michael Wiseman to continue to live, but also forces him to work as an experiment for Dr. Morris and occasionally help fight terrorists. Because of the top-secret nature of the project, Michael is told he may never have contact again with his wife or daughter, under penalty of his own death and the death of anyone he tells of his existence. Despite the threat, Michael finds ways to contact his family while keeping his true identity a secret.
A recurring thread running throughout most episodes is Michael's feelings for his (former?) wife.
