The World War II "Death Railroad", which was build by prisoners of war and Asian laborers under Japanese dictate, did also go through Sangkhlaburi. Remains of the track can be seen in some places, though you need a trained eye and some knowledge of the location. My Sangkhlaburi Map might be of some help, providing you with info on spots where some structures can still be seen.
Best time to explore the track is February - April, as the vegetation is dry and the water level in the lake is at its lowest. Even though the following thought is tempting: DO NOT try to follow the track back from Three Pagodas Pass towards Sangkhlaburi! It runs through Burmese territory and the chance you get picked-up by a Burmese soldier is high.