SRT governor Prapat Jongsanguan said the southern lines was the next target for track repairs after such work for the northern lines were completed.
He said the SRT staff will start a survey on the length of the repair period. Work is expected to begin in May.
Mr Prapat added all southbound train trips would not be suspended, as the repair work will be carried out in stages and not all at once in the same period. He said some rail tracks were in use for a long period of time and trains were unstable when passing through these deteriorating tracks.
The operations will be different to that of the northbound train lines where travelers were forced to travel by road and air as there were repairs in several tunnels.
The SRT governor said about 30 per cent of southern residents preferred travelling by train. The repair operations will be performed only 3-5 hours a day.
The bidding has been opened for private companies to do the job within 900 days, under a Bt6 billion budget allocated by the cabinet under a Bt170 billion improvement of rail track efficiency project.
Following the report of 200 track bolts from the southern line in Thepha district of Songkhla province, Mr Prapat said initial investigation believed it was not the work of terrorists, but local residents who intended to sell the bolts to earn a living.