So I had another bike pull up to me tonight to tell me that my tail light wasn't working. He said my brake light is working but the tail light isn't. I confirmed that this is the case.

Do our tail light bulbs use a dual filament? One for the brake light, one for the running light? If not, what could cause this?

1157 is the most common bulb which puts out 3 candlepower as a tail light and 32 cp as a brake light.

2397 puts out 2 cp & 40 cp if one wants more contrast between tail light and brake light.

2357 puts out 3 cp & 40 cp for that brighter brake light (a good thing).

If an auto parts store doesn't have the 2357, they can order them, probably 2-packs.  These also work well in the front turn signals for brighter lights when signalling.

While you're planting bulbs this spring, you might also search for some brighter replacements for the rear turn signal 1156 bulbs.  Here are some high quality ones from CandlePower.  LEDs are not as bright as an incandescent unless there are many LEDs in the array, and that gets pricey.

I tried the 2357, and went through two bulbs in a matter of three months. Brake light burnt out both times. Went back to the 1157, the original bulb, and been using it ever since. Over a year now, and no failure. YMMV