1. A system for monitoring validators that assess payment tokens, the system including: an input device configured to receive messages from a plurality of interconnected machines comprising the validators, the messages containing data indicative of validator events occurring at the validators, the events comprising at least one of acceptance or rejection of the payment tokens; and a processor configured to: derive timing information for the validator events from the messages; maintain a record of the validator events and associated timing information for the validator events; assess relationships between sets of two or more of the validator events to determine whether predefined criteria is met for any of those sets of two or more of the validator events; perform a specified action in the case that at least one of the predefined criteria is met; wherein at least one of the predefined criteria comprises determining walk-away events at the validators, and determine the walk-away events that represent occasions when an assessment of a token resulting in rejection is followed by a period of greater than a predetermined threshold duration without an assessment resulting in rejection or an assessment resulting in acceptance; and wherein the specified action comprises servicing one or more of the validators according to a prioritization of the one or more validators that is based on the walk-away events at the validators that are identified.