In most other industries, especially transportation-related ones, lack of capacity usually results in higher "fares" - charge what the market will bear to manage demand. If the rates get high enough, they might support adding capacity. Unfortunately, for railroads, the deregulation was only partial so shippers can still force lower rates than the market might bear.

The vast private railroad car fleets we now have are a result of lack of capacity in the past. Car supply was a problem for large, regular shippers. Others got into that game (GATX, TTX, the shippers themselves sometimes). Since the private car lines are NOT regulated or common carriers, they can and do charge what the market will bear and that will let them maintain/upgrade their fleets as needed.

The current problem seems not to be with car supply overall. It's with track capacity, which among other things can limit car supply at specific locations by making delivery difficult. Track capacity is something the railroads have to deal with, but they also have to consider that the oil traffic is probably not long-term. It's a hot game now, but some areas where fracking started earlier are also drying up more quickly than "standard" oil fields. So what do you do if you spend a lot of money to handle the traffic from, say, Bakken then it mostly goes away in 10-15 years. Building new track generally has a longer payback period than that. Though of course you can pull the track out and use the materials elsewhere - but that costs money too. Can rates for oil trains be set high enough to cover all these costs? If so, the current problem will go away in a few years. If not (because of the rate regulations still in place), the capacity problems are here to stay though they might move around to different parts of the network depending on where the high production right now.

When Ports/LALB intermodal traffic was crashing systems about 5 years ago, it was the UP Sunset and BNSF Transcon that took the big hit. That's almost completely flipped. BNSF may not be in meltdown, but the northern and western parts of their system sure look close, due mainly to the oil trains. UP seems somewhat less affected, since they don't have a big piece of the Bakken, but they do get spillover congestion. The Transcon and Sunset, however, have fewer problems since the capacity projects have been (mostly) completed and intermodal traffic dropped off a bit. Amtrak's Sunset, previously the bad boy of their long-distance trains, is now usually on time if not early, while the Empire Builder (a star performer 5 years ago) is now so late that it's sometimes annulled. Moral: when things are bad, they usually aren't bad all over, so be careful that you don't "fix" things in a way that causes other problems.