The train originated in Rosyth, Alta., — right next to Hardisty, which is home to a large oil storage and terminal facility — and was destined for Oklahoma.
It was heading east and travelling at about 70 kilometres per hour when it derailed just after midnight on Monday.
TSB said Wednesday its preliminary examination suggests 19 of the 34 cars that derailed lost its entire load, releasing an estimated 1.5 million litres of crude oil into the ground and atmosphere.
The spill became engulfed in fire, which burned for approximately 24 hours.
“A more precise determination of the tank car damage and the amount of product released will be made as product is recovered and the investigation progresses,” the TSB said in a statement.
It noted the findings are preliminary and subject to change.
The TSB said the derailed cars included a mix of Class 117R and CPC-1232 Class 111 tank cars.
Class 117R cars are an upgraded version considered to have improved safety features over the cars that were involved in the 2013 fatal explosion and fire in Lac Megantic, Que.
The TSB website says in 2015, Transport Canada announced that Class 111 tank cars (including the CPC-1232 tank cars) in flammable liquid service would be gradually phased out.
No waterways appear to be affected, and no injuries were reported. The Ministry of Environment was monitoring the air quality and did not issue any advisories.
Ongoing work continues as the TSB sent six investigators to the site, where they continue to examine mechanical and track components.
Guernsey is roughly 115 kilometres east of Saskatoon.
—With files from The Canadian Press