There are two parts of that assembly that look to me to be typical of K-Line cars of this generation - and indeed Lionel and GGD aluminum-body cars. One is the grooves or slots along the sides that the metal frame slides into; the other is the part of the truck mount consisting of a raised portion of the frame that a tab on the rear of the truck assembly slides into.
Generally there is no option but to take the trucks off before trying to slide the frame out of one end of the car because otherwise they snag on the body skirting. There will be a small screw at one end (toward the middle of the car) that has to be taken out before the tab will slide out to free the truck.
What is not typical is that I cannot see a seam in the end piece of your car. I have long wanted, but do not have, a K-Line business car so I cannot tell you if there is one there. It would not surprise me if there is one but, if not, there must be one at the other end of the car. These are usually press-fitted into the body of the car and in particular at the top on to a kind of t-bar moulding in the roof section. The screws holding the frame to the body pass through this piece, as appears to be the case at the end of the car you've pictured.
One other thing: observation cars of this generation often came with a long screw through the frame into the roof of the rear end piece. If there, it's obvious on looking through the car windows. Its function was to hold the end piece in place.
Final suggestions: (1) the frame sides are a tight fit in the grooves/slots and a small amount of WD40 might be necessary to get it to slide (but very messy around chrome finishes) and (2) the power wires to the lights might be at one end of the car and have to be disconnected to allow the frame to move.
