A traveling duct allows a bonnet to remain over the exhaust pipes of diesel-powered locomotives in motion, and to remain in fluid communication with an Emissions Control Unit (ECU). The bonnet includes a shell with a compliant fender for enclosing the exhaust pipes. One or more of the bonnets are positioned over the exhaust pipe or pipes of the locomotive and are secured to the exhaust pipes or to a top surface of the locomotive. The traveling duct includes an outer duct having a bottom gap or slot. The bonnets may either be connected to an extendable inner duct within the traveling duct, or to a duct transport unit slidably residing within the traveling duct. Exhaust from the locomotive is captured by the bonnet and fed from the bonnet into the traveling duct. The exhaust is then carried by the traveling duct to the ECU for processing.