Don't see CP & CN running joint service in the same corridor especially the Halton Sub that can only support 3 tracks period. More important, you created a nightmare for tank train derailment 3 fold and cutting the network now if such a derailment took place. But most of all, you put are putting people lives at a higher risking of being kill or injure if such derailment took place.
The only way dangerous goods cannot be ship by rail is not shipping them and giving up the good life style we enjoy today.
You will almost triple the number of trains on the Halton Sub and this along raise the back of the residents along that corridor base on future growth.
It make No matter if you relocate both lines to another area, it will have an impact on people living along the line.
GO will run one day on the CP line, but way down the road at this time as there are far more important work to be done first.