When Desoto and Gage are responding to the train fire radio call, as they were driving, they were answering the call to the dispatcher. They were told to respond with lights and siren. You can hear the siren turn on but neither Desoto or Gage reach for a switch to turn on the siren for the squad.
A cut-scene showing the hospital after an emergency with the fire crew shows Squad 51 moving towards the hospital, with a station wagon pulled over. This is the third time in the show that this is viewed. It was obviously an extension of the same scene that was used twice before on other emergencies.
When Engine 51 responds to the call of a girl trapped on a high wire, all four engine crew members are in the cab, as the truck has seats for four. But before arriving, two of the crew are now riding on the back on the truck.
When the Volkswagen hit the back of the truck, there was no police present. The drunk driver was taken away by an ambulance and no police on the scene. The drivers never even exchanged insurance information.