If, after Guy gets shot and then conked on the head in Jodie's apartment, he indeed was found in a coma five days later, his beard would be much fuller. The movie never depicts that stage though and they would have shaved a coma patient.
Guy is told he severed his spine at C6 and was paralyzed from the neck down. In reality an injury to the C6 vertebrae would result in paralysis from the chest down - meaning he'd still be able to move his arms (although finger function would be impaired). However with regards to injuries to C6, or any level for that matter, what is more important is level of nerve damage to determine amount/location of paralysis.
When Detective Burke is about to be hit by the train while caught between tracks in the tunnel, you can see that the train is not a real train, as you can see lights in the background that would be blocked out by the height of the train. This shows that it is either a flat cart with headlights attached, or no more than someone walking down the tracks while holding a light rig to make it appear that a train is bearing down on him.