To enable the smart technologies on the freight railcars, such as the global positioning system (GPS), real-time train condition monitoring and positive train control, a cost-effective power source is required. This paper presents the design, modeling, in-lab and onboard field-tests of an electromagnetic energy harvester for freight railcars. The proposed harvester with a mechanical motion rectification (MMR) mechanism can scavenge the vibration energy that is usually dissipated or wasted. An analytical model considering the train-harvester interaction is established to analyze the dynamic characteristic and predict the performance of the harvesters on different tracks at various train speeds. An in-lab bench test is carried out to experimentally validate the harvester model and evaluate the characteristics of the proposed energy harvester. The experimental results show that an average power of 14.5 W and 9.2 W are achieved respectively for the harvester using 66:1 and 43:1 gearhead under typical suspension vibrations recorded on an operational railcar at 90 km/h. An onboard field test is also performed using the harvester with 43:1 gearhead on a test track, which yields a peak phase power of 73.2 W and an average power of 1.3 W at 30 km/h. Both the in-lab and onboard test results indicate that the proposed energy harvester could continuously generate an amount of power useful for the implementation of smart technologies to improve the operational safety on the freight cars.