I doubt you would get any explosive gases from burning meths,probably some carbon monoxide,which is heavier than air and in a sealed box would ,I guess' put out the fire,(if a lack of oxygen didn't do it first),and a load of water vapour.
Maybe an open topped box with the floor at rail level,and a removable end, raise steam in the box,push the loco out onto the track via the removable end,remove box then add tender and train and off you go.
atb
Phil