This graph and picture of the channel from the Official CRC Navigation Fact Sheet (2MB pdf) pretty well define the problem. All barge traffic is low enough to fit under the high spans of the I-5 bridges, but the high spans do not line up with the swing span on the railroad bridge. During adverse conditions, barges must use the lift spans.
If this problem were solved, you can see from the graph that nearly all bridge lifts on I-5 would be eliminated.
Towboat operators on the Columbia have been trying to solve this problem for some time. They proposed a project to convert the railroad bridge swing span to a lift span, and move it one span south. Their initial proposal (in 1999) for Coast Guard funding to pay for this project was approved at the local Coast Guard level, but rejected at HQ, because most of the benefit went to I-5 users.
In 2004, the towboat operators proposed to JPACT (the regional advisory group for transportation) that this project be moved forward because of its regional transportation benefits. The documents that they submitted give a good history of the project, its cost ($42 million in 2000), and an explanation of the Coast Guard decision and benefits from the project.
See what the towboat operators said in 2004:
Summary of the Vancouver BNSF Rail Bridge Project 1/15/2004 memo to JPACT (267 KB)
Project FAQ (415 KB)
Local Coast Guard District Summary (132 KB)
Highway Benefits (588 KB)
Freight Priority Memo (285 KB)
It is fair to say that the proposal from the towboat operators was tabled in favor of the current CRC process, which still does not solve their problem nor remove the risk to rail freight service inherent in the current rail swing span.
The highest span on the current bridge is 70 feet above water level when the bridge is closed. In 2004 there were only 4 vessels that were taller than that, but they requested the lift span be opened a total of 48 times. There were 6 lifts for vessels in the 60-70 foot range that might have had trouble on a particular day fitting under the bridge because of the water level. The other 550 lifts were for ships that were less than 60 feet tall.
See official CRC "fact sheet" here:
Official CRC Navigation Fact Sheet(2MB)