The number of people coming to Kanchanaburi to see 'the' bridge from the movie "Bridge on the River Kwai" are probably not as numerous as they once were. Still, there are those who still show up expecting to see the gerry-rigged construction of timber and bamboo (even though it was blown up in the movie). The concrete and steel bridge they're presented with is a bit less impressive.
Of course, the truth is that there never was a bridge over the River Kwai. The 'death railway' ran parallel to the Kwai Noi river but never crossed it. The bridge people flock to see is really over the Mae Klong River. After the 1957 movie came out and people began coming to Kanchanaburi in search of 'the bridge', the local authorities renamed a section of the Mae Klong to Kwai Yai.
Not the bridge on the River Kwai
The bridge over the Mae Klong (or Kwai Yai, or whatever) really was part of the death railway, built with prisoner and slave labor by the Japanese during World War II. There were actually two bridges built over the river here. A wooden bridge like you see built in the movie (which was actually shot in Sri Lanka) was built first about 100 meters up-river from the current bridge to expedite construction on the line beyond the river. The concrete and steel 'main' bridge was added during the war when the steel became available. Both bridges were bombed by allied planes near the end of the war. The squarish center spans of the bridge are post-war replacements. The wooden bridge was demolished after the war since its thick structure blocked the flow of the river.
The bridge has steel plates over the ties to make it easy to walk over the span. There are also viewing platforms between the trestles to give you a view of the river as well as to give you a place to get out of the way if you're on the bridge when a train comes.