ULE, I appreciate your longstanding support for Canada, but I think your understanding of this issue needs more reading.

Yes, there was some sympathy with the Confederate cause at the beginning of the war - keep in mind that Americans, mostly from the Northern states, had invaded Canada, burning buildings, etc within living memory.  Nonetheless there was no sympathy for slavery in the Northern terminus of the Underground Railway.  The vast majority of Canadians who fought in the Civil War did so for the North.

It's also a stretch to say we "allowed them to attack the union at St. Albans."  Authorities did not allow cross-border sorties to arrest raiders on Canadian soil, but it wasn't a lot more than that.