You need to expand your vision a bit.
If Keystone XL was approved (currently blocked by GOVERNMENT), then the rail capacity that is currently taken up by oil cars, would be available for crops. It's simple really, and YES, the government is culpable.
While the pipeline was being built, there would still be the same capacity issues with the railroads. The pipeline would eventually end the railroad capacity problems. As long as the pipeline is being blocked, the railroads profit, even though that creates capacity issues.