The firefighters of Firehouse 51 were put to the test when a fiery collision created a life or death situation on the most intense episode of "Chicago Fire" yet.

In "No Regrets," a tanker collides with a commuter train, which then crashes into a warehouse. The big crash provides the central action and focus of the episode. The crew is overwhelmed because victims are trapped, the tanker is about to explode, and supplies are low. The episode shows how each character reacts during the trauma, which gives more insight into each character's personality.

Also during the episode, Boden (Eamonn Walker) makes a life-changing decision about his future. Boden has been dealing with McLeod (Michelle Forbes) and her push for budget cuts, and gave in to early retirement as a way to save the firehouse. He is called in to assist with the train crash, and now wants to come back for good. After getting a negative test result and surviving the building collapse, he is determined to come back and run 51 once again.

As for Casey (Jesse Spencer), he recently found himself the surrogate father of two young boys, but the situation was only temporary. On the episode, he was forced to say goodbye to the kids.

Additionally, Severide (Taylor Kinney) learned more about Kate (Brittany Curran), and Shay (Lauren German) and Dawson (Monica Raymund) are still on the outs.

On the next episode, "Rhymes with Shout," two firefighters will get entangled once again in a dangerous situation. The synopsis reads: "Boden devises a plan to reel in McLeod while Casey and Dawson are caught in a life-threatening situation."

Shay also considers leaving 51, and Dawson and Casey's relationship may begin to change. 

TVLine reports that a new lady will be also romancing Severide on an upcoming episode. "Last Resort's" Daisy Betts will join season two as Rebecca Jones, a fire academy candidate who is taking Kelly's class. Betts' has also appeared on NBC's "Harry's Law" and "Persons Unknown."

Watch the promo for "Rhymes with Shout" below, which airs Tuesday, Nov. 26 at 10 p.m. on NBC.