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 One of the handles is covered by a wooden box, where half of the wooden planks of the original box have been replaced by new ones cut from a tree that fell when nobody was around.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?
My favorite take on the always tells lies/always tells truths puzzle is that the explanation itself was also a lie and both guards will say whatever they think is funniest because guarding is boring work.
I did do that about 800 comics ago ;)