According to Reuters, the stabbing incident was related to a bombing. The counter-terrorism unit of the UK police are currently taking the lead on the investigation. Presently, no "wider threat" has been determined by officials.
The British Transport Police tweeted following the attack that the station will remain shuttered for the duration of their investigation, stressing that "officers are on the scene and a man has been arrested."
Specifically, a woman and two men were injured in the attack, one of whom has been identified as a British Transport Police officer; he sustained a shoulder injury, according to BNO, which reported that the attack took place just before 9 p.m. local time.
— Charlotte Dobson (@dobsonMEN) December 31, 2018
"Just been very close to the most terrifying thing," fellow traveler Sam Clack tweeted. "Man stabbed in Manchester Victoria station on tram platform. Feet from me, I was close to jumping on the tracks as attacker had long kitchen knife."
— ᔕᗩᗰ ᑕᒪᗩᑕK (@Clack_Sam) December 31, 2018