The Brahmaputra Express was on the Down track, on which it was supposed to be. The Up track was clear for the Awadh-Assam Express heading towards New Jalpaiguri. Both trains were travelling at a reported speed of 90 km per hour.

``Due to some reason, the Awadh-Assam Express was allowed to travel on the wrong track by the station master two stations before Gaisal,'' said a Railway official. This is called ``Temporary Single Line operation'' and whenever it happens, all stations ahead are alerted. Also, the driver of the diverted train has to be given ``paper-line clearance.'' In other words, ``somebody from Gaisal station should have gone to the driver and given him in writing that he could proceed.'' Apparently, this was not done.

Otherpossibilities Railway authorities do not rule out are faulty signals and driver's error -- that the driver of the Brahmaputra Express might have overshot a red signal. But this will be difficult to confirm since the four drivers of both the trains were killed.

Yet another reason could be the station master's oversight. Sources say the station master should have been aware that something was seriously wrong, given a facility called ``track circuiting.''

If the track ahead is occupied, it is indicated on the station master's monitoring panel. The signals then remain red, interlocking the track, which effectively means that the series of signals cannot be turned green unless the station master intervenes. The signals can be changed from the relay room which can only be opened jointly by the station master and signal inspector.

Therefore, sources say, the Gaisal station master may have wrongly informed the Awadh-Assam Express or ignored the warning. According to preliminary reports, there was nothing faultywith the track circuiting system.