Heavy Canadian cash crude prices rose on Thursday, extending gains for a fourth consecutive day, as increasing rail loading capacity in Alberta made it easier for producers to ship crude to other markets.

Western Canada Select heavy blend for September delivery last traded at $19.05 per barrel below the West Texas Intermediate benchmark, according to Shorcan Energy brokers. The settlement price on Wednesday was $19.45 per barrel below WTI.

Canadian crude-by-rail loadings rose rapidly to 160,000 barrels per day in the first quarter of 2014, according to the latest data from the National Energy Board, and look set to surge even further by year-end as more dedicated oil train terminals start up.

Producers have welcomed rail as an alternative to congested export pipelines, using it to alleviate the glut of crude in Alberta that can fuel price volatility and leave Canadian grades trading at a steep discount to WTI.

Gibson Energy Inc started loading crude at its new Hardisty, Alberta, unit train terminal in June and is on track to hit full capacity of 140,000 bpd in the fourth quarter.

In a second quarter earnings call on Thursday, Gibson Energy chief executive officer Stew Hanlon said as the company moves increasing numbers of heavy sour “disadvantaged” barrels by rail, volatility should fall.

Meanwhile, Kinder Morgan Energy Partners LP said it would boost capacity at its Edmonton rail terminal, currently under construction, by 110,000 bpd to more than 210,000 bpd by early 2015.

The company also said additional capacity could be increased even further to 250,000 bpd.

“Rail is finally affecting the heavy supply,” said one Calgary-based crude trader.

Light synthetic crude from the oil sands last traded at $2.40 per barrel below WTI, down slightly from Wednesday’s settle at $2.25 per barrel below the benchmark.

On the supply side, Canadian Natural Resources Ltd plans to shutdown production of synthetic crude oil at its Horizon oil sands mining and upgrading project for 25 days from August 16 to complete expansion work.

Horizon produced an average of 119,200 bpd in the second quarter and output is expected to rise to 127,000 bpd after the expansion is complete.