Pneumatic systems in railway applications are vulnerable to water condensation as a result of the cooling of compressed air. This water tends to cause corrosion, degrade lubricants and freeze in cold weather, causing malfunction in brake systems and other pneumatic devices. Prevailing practice for the control of condensation until the early 70’s consisted of cooling the compressed air followed by reduction in pressure to lower the relative humidity. These methods were less than perfect and frozen systems remained a major cause of train delays in cold climates and, in all climates, water continued to corrode devices and emulsify lubricants, increasing maintenance requirements. Regenerating desiccant type air dryers offered a solution. This paper deals with the history of air dryers in railway service, the technologies involved, and the broad significance to the industry.