A woman is in custody after both a man and woman were stabbed aboard the L Line in downtown Denver just before 6 p.m. on Thursday, the Denver Police Department said.

For about two hours, the C, D, E, F, H and L lines were all at a standstill - they were sent to Union Station as officers investigated the stabbing, which happened as the train was pulling into the 18th and California stop, police said.

The trains resumed their normal schedule around 8 p.m. as RTD warned riders to expect delays of up to 30 minutes for the next several hours.

Both victims of the stabbing are being treated at a local hospital for serious injuries, the Police Department said. Neither injury is considered a threat to the victim's life, DPD said.

A fight reportedly broke out on the train and then two people were stabbed. Witnesses gave a good description of the attack, DPD said, and officers found the suspect, later identified as Maressa Christina Archuleta, 26, walking about a block away.

When officers caught up with her, they found a silver knife attached to her pants waistband, according to a probable cause statement by Denver police. She has since been taken into custody.

A witness at the scene told 9NEWS that the woman who attacked the others had been acting erratically on the train. The witness said words were exchanged between the woman and least one of the victims before the woman pulled a knife from her waistband and started swinging. The victim and the witness are related.

The L Line connects downtown to the Five Points neighborhood.