However, I suspect that if the pipeline is built they will still ship via rail if it is profitable - thus the net effect will just be more oil coming from the oil sands.

The only scenario were a pipeline could help the environment (relative to the status quo - it's still bad) would be it rail usage was displaced by pipeline usage. This could be true only when the price of oil holds stable between a narrow range such that pipeline shipments profitable, but not rail shipments. The price is unlikely to ever hold at such a range long enough such that Suncor, etcf.. would adjust production to ship by pipeline only. High sunk capital costs have meant that historically these companies run close to full capacity regardless of price.