The coordinated attacks targeted a passenger train which runs between the city of Quetta and the garrison town of Rawalpindi.
Two bomb attacks targeting a passenger train killed four people and wounded 16 in Pakistan's southwestern Balochistan province on Friday.
The bombs hit the Jaffer Express which runs between the provincial capital, Quetta, and Rawalpindi, the garrison town next to Pakistan's capital, Islamabad.
"The train was stopped, and then as people were leaving the train, there was another explosion under a different (carriage)," said Kashif Akhtar, a senior railways official.
No group claimed responsibility for the attacks but militants fighting the state have frequently targeted transport infrastructure in the past.
Balochistan is the site of China's ambitious $46 billion China-Pakistan Economic Corridor infrastructure project linking its western province of Xinjiang to the Arabian Sea via Pakistan.
Security for Chinese contractors is an issue that the Pakistan government has raised in discussions about the project. In August this year, the ministry of defence said that a 15,000-strong Special Security Division would be deployed for the project.
According to some local reports, the target of Friday's attack were military personnel travelling on the train.
Security forces foiled a similar attack on Thursday as an attacker laid explosives on a railway track near Quetta, Akhtar said.
Railway services from Quetta were suspended after the latest incident.
Pakistani Prime Minister Nawaz Sharif "strongly condemned the attack and directed authorities concerned to find the culprits and bring them to task immediately," an official statement said.
PM #NawazSharif expresses deep grief and sorrow over the loss of precious lives in Mach blasts.— Radio Pakistan (@RadioPakistan) October 7, 2016