OTTAWA, Aug 18 (Reuters) - Canadian National Railway ( CNI )

, one of Canada's two main rail companies, on Sunday

formally notified the Teamsters union that it would start

locking out Teamsters' workers early on Thursday.

"Unless there is an immediate and definite resolution to the

labour conflict, CN will have no choice but to continue the

phased and progressive shutdown of its network which would

culminate in a lockout," it said in a statement.