The agency will also add cars to other trains so that they can handle more passengers, said Ekarat Sriarayanphong, its director of public relations.
With additional cars and trains, SRT will be able to handle up to 100,000 passengers per day during the holidays, Ekarat added.
The state agency will suspend two trains operating on day-trip routes and use their cars for the northern and northeastern routes for the holidays.
The six additional trains are:
Outbound on December 29
1. Train no. 955, departing Bangkok at 10.30pm and arriving in Uttaradit at 7.30am.
2. Train no. 967, departing Bangkok at 9.15pm and arriving in Udon Thani at 8.10am.
3. Train no. 977, departing Bangkok at 9.45pm and arriving in Ubon Ratchathani at 9am.
Inbound on January 2
4. Train no. 962, departing Uttaradit at 7.40pm and arriving in Bangkok at 4.10am.
5. Train no. 934, departing Ubon Ratchathani at 7.30pm and arriving in Bangkok at 6.25am.
6. Train no. 936, departing Udon Thani at 9.10pm and arriving in Bangkok at 7.30am.