The diameter of the hole being drilled keeps getting larger.
It is also virtually impossible to stay alive inside the gas transmission pipeline without some sort of a protection vehicle as the pressure inside is many times (up to a 100 times) the atmospheric pressure. A human would be just crushed by such pressure. There is no way one could just 'open a hatch' and step inside.
It is impossible to move inside a gas pipeline without the pipeline operator immediately noticing it, since the pressure in the pipeline is monitored in minute detail 24/7.
The trains used in the channel tunnel are driven electrically (the current collector on top of the locomotive is clearly visible). Yet, in all exterior shots of the train moving before entering the tunnel, no electric lines are visible.
At 0:42:25, an explosion that shatters safety glass would cause hearing loss.
Due to Operating and Environmental reasons, there are sensors that monitor pressure and flow rate continuously on all cross-country piping and the first time there was a minute pressure drop in a pipeline of that size, it would immediately shut down the pipeline.
At 0:34:53, Tom spots Grace Lewis and then phones Declan at the SAS Command telling him that she is on his train. Declan tells Major Bisset, who says: "Before I do anything, I need eyes on Grace." Tom has already provided the information, but Bisset obviously wants an independent confirmation.
Multiple times somebody is lying on the roof of the train, hiding or shooting. It is quite possible that that person would have been killed by electrocution due to a flashover from the electric overhead line.
It seems like a bad idea -opening the gas pipe line when there are so many fires burning near by.