The threat of Sydney's rail network grinding to a halt next week remains "on the table", according to the Rail, Tram and Bus Union after weekend talks failed to resolve the impasse over train workers' pay and conditions.

While both sides expressed optimism that a deal could be reached this week to avert the planned strike, weekend crisis talks did not break the deadlock in negotiations.

As the clock ticks toward next Monday's 24-hour strike which is expected to cripple the city, an earlier strike could cause extra havoc on the rail network.

On Thursday night, the day before Australia Day, the first round of protected industrial action will begin with staff refusing to work overtime from midnight.