The Khok Salung Railway Bridge is on the Bua Yai - Kaeng Khoi train line. It crosses the water reservoir created by the Pasak Chonlasit Dam. The station and the railway line were built after the old one was submerged in water after the construction of the dam. Apart from the long excursion trains departing from Bangkok, the passenger convoys that use this line are quite short because they carry few passengers.
Personally, apart from taking one of the train excursions, I don't think it's a must visit in Lopburi ! But if you're not far, why not make a detour ?
For the 2023 season, the train excursion is expected to depart from Bang Sue Grand Station around 6 a.m., stop at at Samsen, Bang Sue, Bang Khen, Lak Si, Don Mueang and Rangsit and Ayutthaya Junction, Saraburi and Kaeng Khoi before reaching the bridge at around 10:30 am and stopping there for 20 minutes to let tourists to enjoy the scenery and take photos. Then, it will continue to the last station (Khok Salung) where it will stop for 30 minutes for tourists to shop local products. Then it will return to the dam so that tourists have free time and can have lunch. The return trip will leave at around 3:30 p.m. to arrive in Bangkok a little before 7 p.m.