King of the Hill S12 E19 Strangeness on a Train

"Return of the king."

Stressed that her birthday parties never live up to her expectations, Peggy plans herself a disco-themed party on a mystery train, but after Luanne ruins the surprise, Peggy worries her parties will always end badly. However, a new mystery arises after a couple sneaks away for some private time in the train bathroom, and the partygoers must figure out who did it.