Stephen Harper's summer schedule for major political moves appears to have been thwarted yet again by an unforeseen catastrophe.

Conservative sources say a major cabinet shuffle that was initially pencilled in for mid-week has been pushed back, likely to next week, following the rail tragedy in Lac-Megantic, Que., which killed at least 15 people.

The Prime Minister's Office refuses to say when the shuffle will take place.

A freight train laden with crude oil derailed in the centre of town and exploded into a series of towering fireballs, obliterating a huge swath of the town's core, including a popular watering hole.

Transport Minister Denis Lebel and Industry Minister Christian Paradis, whose riding encompasses Lac-Megantic, have both been on the scene in recent days.

It's the second time Conservative plans have been thwarted recently by outside forces: the party's policy convention was postponed in June following floods in southern Alberta.