Cigarette's do not cause a fire of this magnitude. A burning butt can burn a hole on the seat or any cloth but can not start a fire. To start a fire there has to be at least a reasonably strong spark . Fire of these nature are generally caused by fireworks, electrical short-circuit, flame on an inflammable materiel etc. Since maintenance of railway coaches are very bad, the probability of the fire having been caused by short circuit is high. Probably during investigation, they may also check up if any cleaning of the toilet was done at the station.. In that case there is a possibility of fire having been caused by a smoker. if anyone had entered the toilet immediately after the cleaning was done and after lighting a cigarette had thrown the burning matchstick down, then it is possible the burning stick could have reacted with the cleaning lotion like phenol and could have started a fire. The probability of this is less. Even in big station railways don't bother to carry out cleaning.