United Liberation Front for Asom militants on Sunday blew up railway tracks in Sibsagar, minutes after a Rajdhani Express passed, police said.
They also damaged an oil pipeline in Dibrugarh.
The banned outfit, though, denied any hand in the serial blasts that have rocked Assam since Thursday.
The remote-controlled bomb exploded on Sunday morning between Bhojo and Sibsagar stations after the Dibrugarh-Howrah Kamrup Express and Dibrugarh-New Delhi Rajdhani Express passed.
There was no casualty though railway tracks have been extensively damaged and rail traffic has come to a halt with several trains controlled at various places.
Earlier, on Friday night, railway tracks were damaged following a blast at Balimar near Naharkatia in Upper Assam just before the Delhi-bound Rajdhani was to pass by. The gas pipeline in the area was also damaged in the blast.
An oil pipeline was blown up last midnight at Kothaloni under Tengakhat police station in Dibrugarh district.
The militants also blew up two pipelines in Tinsukia and Dibrugarh district on Saturday night, affecting oil and gas supply to Brahmaputra Valley Fertilizer Company Limited.
Altogether six people have lost their lives and more than 40 people injured by blasts since June 8.
The ULFA, however, denied its involvement in the blasts and blamed certain vested interests for these incidents.
ULFA Commander-in-Chief Paresh Barua in a statement to the local media accused vested interests of masterminding the blasts to derail the peace process and defame the outfit.
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