By Euan Rocha

TORONTO, Oct 14 (Reuters) - Talks between the railroad workers' union and Canadian National Railway Co have broken down, raising the possibility of a strike or a lockout by Oct. 28 at the country's largest railroad operator, the union said on Monday.

The Teamsters Canada Rail Conference (TCRC), which represents some 3,300 conductors, trainmen, yardmen and traffic coordinators, said the company rejected the union's offer to extend conciliation talks that ended on Oct. 7.

"We're extremely disappointed by CN's refusal to extend the mediation period," Roland Hackl, a spokesman for the union, said in a statement.

The union said the talks have stalled over CN's demands for concessions that would force workers to work longer hours with less rest time between trips. Wages and the retirement plan are not central issues in this bargaining round, the union said.

The union said the company's demands fly in the face of its statements about safety being a priority.

A CN spokesman said the company does not comment on the substance of ongoing labor negotiations, but stressed that CN's bargaining proposals would not in any way compromise the health and safety of TCRC members.

CN remains optimistic it can negotiate an amicable settlement with the TCRC to avoid labor disruption in Canada, the spokesman, Mark Hallman, said in an email.

Hallman also noted that the company has comprehensive safety programs in place designed to prevent accidents.

Rail safety has become a hot-button issue in Canada this year after a disaster in July when a runaway tanker train derailed and exploded in the middle of the small town of Lac-Megantic, Quebec, causing 47 deaths and hundreds of millions of dollars in damage.

The accident prompted Canada to toughen its safety standards for railways earlier this year.

Last May, the Canadian government stepped in to force an end to a strike that shut down the freight operations of Canada's No. 2 railroad operator, Canadian Pacific Railway Ltd, as the strike impacted shipments of grain, coal, fertilizer, autos and other goods.

The strike at CP last year occurred after 4,800 unionized locomotive engineers, conductors and rail controllers walked off the job over the company's plan to cut pension payments.

It is unclear whether the federal government would step in and pass back-to-work legislation if a strike or lockout occurs at CN Rail.