Class S12 is a diesel multiple-unit (DMU), built for Sri Lanka Railways by China's CSR Corporation. The first batch arrived in Sri Lanka in August 2012. They were built to replace locomotive-hauled passenger trains. Seven of the S12 DMUs were ordered to strengthen long-distance travel on the Main line from Colombo to Badulla. Four of S12s will serve the Kelani Valley Line. The remaining two designed as luxury trains.^[1]

The S12 DMUs were meant to improve services on the Main Line and the Kelani Valley Line. The steep slopes on the up-country portion of the Main Line is difficult to operate. Prior to the S12, only a few models of locomotives were capable of operating this line. As DMUs started to replace locomotive-operated trains on other lines, the up-country Main Line could not be operated by DMUs. The S12 is the first DMU to handle these conditions.

The locomotives are purchased under a USD 101 million credit line agreement with China. The loan would be settled within 15 years.^[2]

In September 2012, the first units had trial runs.^[1] The first Main-Line train using the S12 began on 23 October 2012.^[3]

Main Line sets

• Colombo Fort - Kandy / Matale - Badulla

Office sets

• Colombo Fort - Polgahawela - Rambukkana
• Colombo Fort - Negambo
• Colombo Fort-Kosgama

Luxury sets

• Colombo Fort - Anuradapura - Pallai
• Kandy - Ella