The police have zeroed in on the modus operandi of dacoit gangs operating in Jharkhand that rob train passengers at gun point.The operation is simple, officials said. As the trains move out of Bihar, armed dacoits board in Jharkhand and rob passengers when entering West Bengal. After committing the crime they pull the chain and escape.The dacoits, in masks, target general bogeys since all sleeper and AC coaches are escorted by RPF or General Railway police, officials said.Last week, the West Bengal Police arrested eight persons in connection with the dacoities in the Hatia-Burdawan passenger on June 16, and the Vananchal Express on June 3.All arrests took place in West Bengal’s Purulia district, officials said. The accused are yet to be interrogated by the Jharkhand GRP.Jharkhand IG (Rail) I Lakra said: ‘‘We are going to take them on remand for further interrogations.’’On the morning of June 24, another alleged gang entered the Ranchi bound Patliputra Express at Bokaro, robbed almost all passengers in the general bogey between Kotshila and Jhalda, in West Bengal, and fled after pulling chain.After the train stopped at Muri, West Bengal, Rakesh Kumar, a victim, lodged FIR at the GRP station. However, 40 hours later, Lakra said since the site of crime falls in Jhalda, the FIR was being sent to West Bengal GRP for registration.