Canadian crude oil exports by rail in the first quarter fell 28 per cent year on year because more moved by pipeline, said Canada's National Energy Board, Reuters reports.

Shippers exported on average 119,755 barrels per day of crude by rail to the United States in the first quarter, down 24 per cent, according data from the regulator.

ARC Financial analyst Jackie Forrest said rail volumes dropped because Enbridge Inc, whose Mainline system ships the bulk of Canadian crude exports to the US, added 340,000 bpd of capacity through debottlenecking and expansion of the pipeline.

"The biggest thing driving the rail volumes is the actual supply versus the amount of pipeline capacity we have," Ms Forrest said, adding that because pipeline is cheaper the preference will be to fill the pipeline before turning to rail.

Total Canadian crude supply has also declined to around four million bpd since hitting a peak of 4.6 million bpd in January, according to a Barclays research note, as a result of maintenance in the oil sands patch and recent wildfires in Alberta.