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KiwiRail South Island operations general manager Mark Heissenbuttel said the two pieces of equipment had aged and had to be replaced.
They were used on the main south line, accessing the freight yard next to Dunedin Railway Station, "making them crucial" to train operations.
Replacement work began on Monday and would continue over the weekend, he said.
Trains would continue to run, but the facilities being replaced would be fully operational on Monday afternoon, after the signalling equipment was re-established.