British Transport Police (BTP) arrested two men during a series of dawn raids.
A total of 17 officers swooped on properties in Dowlais and Fochriw, as well as the Merthyr Metal Recycling scrap metal dealers in Merthyr Tydfil as part of Operation Leek.
A 36-year-old man from Fochriw was arrested on suspicion of theft of railway cable, while a 48-year-old man from the Merthyr area was arrested on suspicion of handling stolen goods and possession of a controlled drug.
Various items were seized during the raids, which happened on Wednesday August 19, including cash, a quantity of cannabis, mobile phones, laptops and railway property, including plant and cable.
Operation Leek is the culmination of a three-month investigation by BTP into the theft of railway cable in the Theale and Thatcham areas of Berkshire.
Officers were joined by colleagues from HM Revenue and Customs, Merthyr Tydfil County Borough Council, South Wales Police and the Health and Safety Executive.
Detective Constable Dan Murdoch, of British Transport Police, said: “With three search warrants executed simultaneously at properties in Merthyr, Dowlais and Fochriw, this was a very successful operation for all agencies involved.
“The theft of railway property causes severe disruption to the everyday lives of the travelling public and has huge financial implications for the local economy; it really is an attack on the community.
“Each year, the action of thieves costs the industry hundreds of thousands of pounds in repairs and results in countless delays and cancellations, along with huge associated compensation costs.”