This paper presents new results on the mechanics of single Wheelset wheelclimb derailment and associated safety criteria, and applies them to complete rail vehicle trucks. Excellent agreement between theory and experiment for single wheelsets is demonstrated for quasisteady wheelclimb and for conditions where incident lateral velocity and yaw angle variation are significant. A piecewise linear, quasisteady model for derailment of a complete truck is developed, which demonstrates that lateral forces are generally distributed unequally between the axles in a truck. Analysis of truck derailment is shown to require careful analysis of the displacement and loading of each wheelset, with truck-based derailment criteria inapplicable to the wheelclimb mode. Preliminary experimental results using a one-fifth scale model of a three-piece freight car truck are used to identify principal modes active in derailment.