Practical information about River Kwai Bridge
Try to be there when the train passes on the bridge, well, not on the bridge (even if it is possible, there are spaces provided to park on the side of the track) but on the river bank to see the train pass. I also advise you to cross the River Kwai Bridge completely. There is a large Chinese temple and the tomb of the Chinese soldier to visit a few minutes walk on the other side.
To go visit River Kwai Bridge, you can also book one of the tours above.