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OTTAWA, Aug 30 (Reuters) - The union representing

workers at Canada's two main rail companies said on Friday it

had filed four court challenges against a ruling by the

country's industrial labor board that forced them back to work.

The board on Saturday accepted a request from the government to

order more than 9,000 Teamsters members back to work at Canadian

National Railway ( CNI ) and Canadian Pacific Kansas City ( CP )

and to impose binding arbitration.

The union said the decision was a win for the railways and could

lead to the imposition of future contracts, eroding workers'

bargaining power.

Federal Labour Minister Steven MacKinnon said last week his

decision to refer the matter to the board would survive a court

challenge given his broad power under Canada's labor code.

Government officials say the challenge in the Federal Court

is separate from the board's ruling and will not affect the

return to work.

The two companies locked out the workers earlier this month

after talks failed to produce a new labor deal.