Tham Krasae (Krasae Cave) was used by the Japanese while building this difficult stretch of the Death Railway. The wooden trestle bridge they ordered the prisoners of war to build, called the Wang Pho Viaduct, snakes between the river and sheer cliffs and is still used by trains today.
The gauntlet of souvenir shops detracts from the experience a bit, but it's still an impressive and beautiful place – especially when passenger trains pass by (from the south at around 7.15am, 11.45am and 5.45pm and from the north about 6am, 1.30pm and 5.15pm). Walking over is allowed (though not advisable for those afraid of heights) and the trains cross very slowly so you have time to get out of the way when you hear them coming.
The cave now has a large Buddha image in the main chamber and a small King Rama V statue at the back.