Talk in safety-critical activities displays features that distinguish it from both ordinaryconversations as well as from other institutional talk, but it also shares some features with these.Formality and informality are both interactionally accomplished phenomena, but they are shapedthrough different sources. Safety rules and pre-printed forms constitute two sources offormalization, dictating how to carry out communicative exchanges in certain types of situations,irrespective of the more specific circumstances in individual cases. Sources of informalization arethe participants’ need to adapt to situation-specific communicative needs, but also, ironicallyenough, routinization itself.In contemporary literature, safety-critical talk tends to be treated either in terms of strictadherence to a formal code, where all informalities are seen as potential sources of accidents, orinformalization is treated as natural and inevitable, focusing on routine conditions where they areapparently harmless. In this paper, based upon detailed analysis of telephone calls between traindrivers and dispatchers on the Swedish railway network, we propose a middle ground. We suggest acontingent theory of formalization, identifying four main types of informalizations, as well asdiscussing when and why they may be harmless and when they may be detrimental.