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 One of the handles is covered by a wooden box, where half of the wooden planks of the original box have been replaced by new ones cut from a tree that fell when nobody was around.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?
Sounds like we’re making a lot of modifications to the setting here, but that would require the other guard to also not know because he actually doesn’t. If either of them makes a definitive answer which you can then verify, then you can solve it.