Patna, April 13: Next time someone asks what good is a Re 1 coin these days, tell them it can help loot a train.

Senior government railway police (GRP) and railways officials are stunned after the interrogation of four suspects in the April 9 robbery on the Patna-bound Rajdhani Express revealed that the gang had used a coin to bring the train to a stop at Gahmar railway station, around 151km west of Patna and close to the Bihar border in Uttar Pradesh's Ghazipur district.

The suspects, nabbed from the UP-Bihar border areas late on Wednesday night, claimed they used a Re 1 coin to "hack" the outer signal of Gahmar station into turning red so that the train would stop and they could board it.

The railways build circuits on the tracks close to the signals to make them turn green or red. Train tracks are made of 13m-long pieces of metal. There is a little gap between two pieces where they join. An insulating material is filled in this gap to break the circuit so that the signal turns green, one of the four arrested men explained - to the investigating team's utter astonishment.

A train's metallic wheels act as a conductor between the gap, helping the signal turn red so that another train cannot come on the same track from behind.

When the Rajdhani was about to reach the Gahmar outer signal, a gang member put a one-rupee coin in the track gap ahead of the signal - the circuit was completed, the outer signal turned red and the driver had to stop the train. Another gang member, who was on the train, opened the doors of a coach and the robbers looted passengers on the A4, B7 and B8 coaches.

The gang, it is learnt, looted other trains - in Buxar and Ara in Bihar and Allahabad, Jaunpur, Ghazipur and Varanasi in Uttar Pradesh - using the same modus operandi.

A senior GRP official said a joint Bihar and Uttar Pradesh GRP team arrested the four suspects: Om Prakash, Sawan Baba, Fateh Hussain and Raja Kumar, all residents of Ghazipur. The police team, headed by Gaya GRP inspector Sunil Kumar, carried out intensive raids in areas close to the Bihar-UP border. Two more men from Buxar have also been detained for interrogation, an officer said.

The cops have recovered some of the jewellery and cellphones the gang had snatched from train passengers. "While four members of the gang have been arrested, others are still at large," an officer said.

East Central Railway chief public relations officer A.K. Rajak said the Rajdhani probe was still on, but that the coin hack was possible.

A senior railways engineer also said the coin hack was possible in theory.