The British Transport Police said the line of enquiry centred on the arrest of two men 30 miles north of Potters Bar nine days before the fatal accident, which occurred last Friday just before 1pm.
The arrest occurred after an off-duty policeman walking his dog next to track near the village of Little Wymondley, north of Stevenage, heard explosions. He called police after seeing two men setting light to aerosol cans. It later transpired that they were rail security guards.
A transport police spokesperson confirmed that the incident was being looked at but would not be drawn on the possibility of a link with the Potters Bar crash, in which seven people died. The transport police are investigating the incident with the Health and Safety Executive.
The spokesperson said: "This information is known to the [Potters Bar] inquiry team. There is no reason to believe it is connected to the Potters Bar crash but the team has not looked at the case in detail yet."
Rail sources have highlighted two incidents in which points were damaged deliberately. One was at Darlington, County Durham, in 1998, and the other was at Thirsk, North Yorkshire, in 1999. A source said: "This is proof that these kind of incidents have been happening. At this stage it [sabotage] cannot be discounted."
Rail sources have highlighted two incidents in which points were damaged deliberately
The incident at Little Wymondley took place on the same day that a three-man Jarvis team found two loose nuts at the Potters Bar points. These nuts, and two others, were found on the ground after the accident.
The investigation into the Little Wymondley incident comes despite the initial findings of the HSE inquiry, which concluded that a points failure was the likely cause of the accident.
It said it had found no evidence that vandalism or faulty signalling was to blame for the crash. Investigation manager Frank Hyland said: "It would have been extremely sophisticated and daring to be vandalism." The HSE said "one or two handfuls" of nuts had been tightened as a result of checks on other points after the crash.
Hyland said it was very significant that Jarvis had discovered discarded nuts nine days before the accident. One theory put forward by the HSE is that the four nuts had come away because of vibration.