The oil doesn’t stay in the ground, it gets shipped via diesel trains without the pipeline.

I’m guessing climate change is the reason, but it’s hard to see how the current state of affairs is better for the environment. Diesel trains are a lot less carbon friendly than pipelines.

Here's a thought experiment: if the diesel train works as well as the pipeline, why build the pipeline?

It doesn’t work as well, it costs more. But it’s still better to use the train than to slow production

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: if a diesel train produces no pollution, why not use it everywhere?

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.