BUSAN (Yonhap) -- A KTX bullet train abruptly stopped for an unknown reason inside a tunnel shortly after departing from Busan for Seoul on Sunday, paralyzing Seoul-bound railway traffic for about an hour, railway officials said.

   The train came to an unscheduled halt at 12:13 p.m. inside the 20.3 kilometer-long Geumjeong tunnel on the outskirts of Busan, the longest in the country.

   The stoppage forced about 500 passengers to return to Busan Station via another KTX train, said the Korea Railroad Corp. (KORAIL), the operator of the KTX trains.

   "We are now trying to determine the cause of the incident," a KORAIL official said.

   It was the latest in a series of incidents involving KTX trains.

   On Feb. 26, a KTX-Sancheon train, a newly developed Korean-style model, encountered an engine problem after departing from the southeastern city of Daegu, and arrived at Seoul Station 39 minutes behind schedule. The incident occurred just one day after a KTX train came to an abrupt stop due to a heat sensor malfunction on the Busan-Seoul route.

   And on Feb. 11, another KTX-Sancheon train derailed near Gwangmyeong Station, south of Seoul, due to a combination of human mistakes, including the loss of a nut in a device that controls track movement for trains.

   The train involved in the latest incident, however, is not a KTX-Sancheon, the official added.