The decay rate of rail vibration as a function of the distance from the excitation source is a key factor determining the general rolling noise contribution from the rail. A higher decay rate results in a shorter effective rail radiating length and therefore less noise emission from the rail. For the purpose of estimating rail noise radiation and evaluating the effectiveness of rail damping measures, the decay rate has to be determined accurately. The conventional impact hammer approach requires several hours of on-track time and normally manual data processing. An alternative method by using a passing train as the source of excitation requires track access to attach the sensors only and data processing can be automated. Results from the two methods have been compared for two elevated track situations. It was discovered that there was significant variation between the results obtained from the two methods. The reasons for the discrepancies may include the effect on the damping characteristics of the rail fastening system of the train's presence. The train pass-by method shows a good consistency over different train pass-bys.