But costing more and having limited capacity most certainly does slow production!

Are you saying that the trains have the same throughput and cost as the pipeline would have?

The pipeline unquestionably means more production, more co2.

Here's another observation (not a thought experiment): To figure out which scenario pollutes more, one could add up the various inputs and outputs. A producer is limited by the volume and cost of different means of transportation. It isn't apodictically true that the same volume of tar is delivered regardless of the means, so preventing a pipeline from being built does not necessarily increase net pollution. That being nebulous, the Democrats' motivations are less nebulous.