Canadian Pacific says while it initially thought only four barrels of oil has escaped the toppled cars, it appears about 400 barrels broke free.
Some 20 cars — including two that contained light sweet crude — went off the tracks Wednesday morning near White River, about halfway between Thunder Bay and Sault Ste. Marie.
CP spokesman Ed Greenberg says crews quickly contained a leak in one of the cars but didn't realize the other had spilled until much later Wednesday.
Greenberg says the spill was initially hidden under the snow and migrated "a short distance" beneath it.
He says CP (TSX:CP) has built a berm and taken other steps to contain the leak, and all the oil will be removed during the cleanup.
"CP takes these incidents seriously," he said in a statement.
"Soil and ground water sampling around and below the site is being conducted today. There is no indication from any of the sampling sites that the product has migrated beyond the containment berms," he added.
Teams from the Transportation Safety Board and the province's Ministry of the Environment are also on site to help with the investigation and remediation.
Greenberg says train operations through the area are expected to resume Thursday evening.
Also on HuffPost