The Rajdhani Express connects India's northeast with the capital. It can carry 900 passengers, but was not running at full capacity, Rabha said.
All of those killed in the blast were migrant workers from Bihar, the Hindustan Times website quoted a police offical as saying.
"We suspect the bomb was planted inside the train from early examinations, but we cannot conclude for sure just now," P Saloi, a police superintendent, said.
Several armed groups, including the separatist United Liberation Front of Asom, are fighting for autonomy or independence in the Assam region, home to 26 million people.
The fighters say India's national government exploits the northeast's rich natural resources while doing little for the area's indigenous people.