Three of the dead were children and another 20 people were wounded by the blast in Kashmor district, which caused several train cars to run off the track, said police official Mohammad Azeem.
Kashmor is in Sindh province but adjoins insurgency-hit Baluchistan. No one has claimed responsibility, but suspicion likely to fall on ethnic Baluch separatists operating in the resource-rich province. Al-Qaida-linked militants also have a presence there.
One of the separatist groups last week blew up three gas pipelines, suspending supplies to millions of household for two days. The group said it bombed the pipelines to avenge several Baluch whose bodies had been found in the Khuzdar district in the region.
Meanwhile, a roadside bomb killed a policeman guarding a polio vaccination drive team in the outskirts of northwestern city of Peshawar. Police official Ibrahim Khan said another officer was wounded in the attack during the weekly anti-polio campaign in the Budhni area.