Detective Superintendent Sam Blackburn, a senior investigating officer for British Transport Police, said: "Stephanie was a young woman with a whole life ahead of her to enjoy.
Jason Flore was seen arguing with 19-year-old Stephanie Marie just moments before he plunged a 20cm blade through her heart.
The couple, who shared a child, were seen rowing on a railway bridge as Flore acted in an "aggressive" manner towards Stephanie.
Prosecutor Patrick Dennis said: "The defendant said he was friends with Stephanie Marie, not her partner.
"This outcome is the result of countless hours of investigation and I thank everyone who has contributed to ensuring Stephanie's killer has been held accountable for what he did today.
