The policy debate over the Keystone XL pipeline (as opposed to the political one) in many ways boil down to railroads. Supporters of Keystone say there's no point in blocking the construction of massive pipeline that will traverse the United States, since the projected 510,000 barrels of oil the pipeline would transport could just be moved around on trains. Either way, they say, the oil is coming out of the ground and it's going to be burned.

However, there is reason to think that transporting a monstrous influx of new oil by rail would be a lot more difficult than many imagine.

It's not because the U.S. freight rail system is a failure. Our passenger rail is a complete joke and a national disgrace, but our freight rail is, by most accounts, quite good. However, like everything about American infrastructure, it is very far from perfect. More importantly, it is already jammed to capacity.

A recent story about food shipments illustrates this point. With the explosion of domestic fracking, energy transportation, which is more lucrative than food transportation, has been pricing farmers off the rails:

The furious pace of energy exploration in North Dakota is creating a crisis for farmers whose grain shipments have been held up by a vast new movement of oil by rail, leading to millions of dollars in agricultural losses and slower production for breakfast cereal giants like General Mills. The backlog is only going to get worse, farmers said, as they prepared this week for what is expected to be a record crop of wheat and soybeans. [New York Times]

What would happen to the U.S. freight rail system if you added another 500,000 barrels per day? Perhaps it would bounce off other commodities. But perhaps the oil itself would end up facing a huge bottleneck, trapping a lot of of it in Canada.

Indeed, despite its overall decent quality, American freight rail is plagued by many longstanding snarls and headaches. Chicago, the rail capital of the nation, is notorious for this kind of thing. For decades, shipments from New York have taken longer to get across the city than to get there in the first place. Only in the last year or two have Chicago authorities got going on the desperately needed throughways and bypasses to make the city a half-decent rail hub. They’re looking to keep upgrading, but unsurprisingly financing is proving to be a problem. As recently as March of this year, companies were shipping goods all the way to Arkansas to avoid the Chicago tangle.

Furthermore, total freight rail shipments have grown fairly slowly: from 1.60 billion ton-miles in 2001, to 1.73 billion in 2011 (the most recent year for which numbers are available).

There are other factors to consider as well. Farmers are politically influential and may try to slap additional safety regulations on oil trains to reduce their price advantage. With 2013 a record year for oil train crashes — one such explosion in Canada last year killed 47 people — that’s probably worth doing on its own.

Thanks to the jankiness of American infrastructure, it turns out that rail is not an ideal Plan B for the oil industry. That makes an anti-Keystone strategy a decent rearguard action to keep some of that oil in the ground where it belongs.