Orange I Glad?

I wrote this for the latest round of NPR Three-Minute Fiction. The criteria was that it must contain a joke, and a character must cry. It had to be done in six-hundred words or less, and because I can't seem to get that brevity is the soul of wit, mine clocks in at five ninety-six.
It's past the end of the world and it contains a very bad joke, but which one it is, is for you to decide.

 He used knock-knock jokes like cadence calls, keeping one foot moving ahead of the other as the two of us, road-worn travelers shuffled passed Victory station on the old blue line.

“Knock-knock,” his words cut through air choked with the detritus of disuse as he danced ahead of me on what had once been the deadly third rail, just to prove that he could.

“Who's there?” my voice was phlegmy and distant in my own ears, toneless and mechanical, but it was a voice and that's more than most people could still claim, these days.

“Banana.” it was this one again, he must be getting tired.

“Banana who?” left foot step, right foot step, wince as the thin spot on the soul of my boot strikes something in the dark, left foot step.

“Knock-knock,” we're almost there, I didn't need him anymore, not really, I was behind him and covered by darkness. There was only one way to shut him up, but I had done worse...

“Who's there?” I have some honor left, he's helped me this far, and that's not nothing.

“Banana,” the tunnel is an old one, like all those that are still usable, brick arches weathered the blasts better than cheap steel beams, but it's not as old as the joke feels now and much more beautiful.

“Banana who?” the rail map I'd passed so many times one the walls played itself forward in my head; Victory station, Denmark station, Providence, then a sprint through the lights still powered by some ancient back up generator to the mouth of the orange line, then Patriot, Loyalty, and out at Triumph station. If my information was good there was a club there, called the Kellar. I haven't sung since the bombs dropped, not for an audience at least, but I dropped that stubborn five pounds...

“Knock-knock,” God let it be over.

“Who's there?” The orange line was much newer, and commensurately more difficult to navigate, but it was still safer than trying the surface. Cooler too, in more ways than one.

“Banana,” we did see light for the first time in I don't know how long and I can't complain about that.

“Banana who?” Close now, up the stairs, two at a time despite our fatigue. Enter the lobby guns drawn, cover each other like we've gotten so used to doing, one more flight of stairs, one more arch.

“Knock-knock,” a hundred feet from our goal, if my information is right, and I damn near killed him anyway. I took a deep breath instead.

“Who's there?”

“Orange,” He was grinning like a mad-man, the mousy man, boy really, I'd picked up outside of Chicago. For the first time I noticed the fever behind his ever-present grin, and the fear.

“Orange who?”

“Orange you glad I didn't say banana?” Even he lacked the gall to laugh. We opened the doors as one.

The flickering neon sign across the road was the most beautiful thing I had ever seen, the lights in the security windows a close second, and I rushed across the street only to find myself alone, I turned back to see him standing in the mouth of the train station, tears streaming down his face.

“What's your name?” He called to me.

“Sally,” I replied with a wink, and, devil be damned, I continued, “Sally Bowles.”

“Still making jokes,” I heard him murmur, as he turned away, and slipped back into the tunnels.