Both of the guards have a handle. If the guard that always lies pulls his handle, then the minecart will divert to the second track. If both guards pull their handle, the prisoners will be released. One of the handles is covered by a wooden box, where half of the wooden planks of the original box have been replaced by new ones cut from a tree that fell when nobody was around. If guard A is taller than guard B, are we getting paid for this quest?
The part about not being able to see the sun is almost certainly true, though, and the usual premise of the riddle is that one of them ONLY tells the truth and one ONLY lies.
As such, either you're in something as implausible as a dungeon with a skylight or that guard is the truth teller.