The teen was critically hurt when the car she was a passenger in collided with a tram at around 6.30am on Thursday morning.
She was initially said to be in a life-threatening condition and on Friday police said she was critical but stable.
Meanwhile the teenage driver of the car, who is suspected of drink driving, remains in hospital as a precaution.
He was due to be taken into custody "as soon as possible," West Midlands Police said.
The West Midlands Metro tram and the Vauxhall car the teenagers were in were both badly damaged in the collision, which happened at the Bilston Street Island in the city centre.
Nobody else was seriously hurt but a passenger on the tram suffered minor injuries.
The nearby road was shut for much of Thursday as collision investigators examined the scene and the vehicles were recovered.
Meanwhile trams were back running to the end of the line at nearby Wolverhampton St Georges by lunchtime on Thursday.
It is the fourth time in four years that serious crashes involving trams and cars have happened at the roundabout, which is next to the old Royal Hospital and Wolverhampton Crown Court.
The tram line run throughs the centre of the island which connects the ring road, the A41 Bilston Road, Bilston Street into the city centre and the A454.