The two men fled after threatening the three workers, who spotted them while checking the line near the steelworks in Cambuslang Road.
The thieves, who escaped with a small amount of cable, ran off along a footpath beside the River Clyde towards London Road.
One is described as 6ft tall, 25-30 years old, of medium build and wearing a black tracksuit with a hood and gloves.
The other man is described as about 5ft 7in tall and balding. He was wearing a green fishing-style jacket and was carrying a black rucksack.
BTP said the incident happened about 5:45pm on Wednesday, 1 April.
Detective Constable Kevin Foulis said: “I am anxious to speak with anyone who was in the area at the time and saw the two men responsible.
“Anyone with information which can assist our enquiries is urged to contact BTP on 0800 405040 or by texting 61016.”
The incident comes as cable theft on Scotland’s railways is falling, with BTP saying it has halved in the last two years.
It is also believed to have been a very rare case of violence being threatened.
This is partly because most thefts have happened in remote locations in the dark.
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