Legally passenger gets priority, however to increase the already disgusting profit margins and decrease capital expenditures (why spend on infrastructure when you can pay dividends and do stock buybacks), they have been running longer and longer trains without increasing the length of sidings so when a passenger train wants to pass a 2 mile long freight train, there's literally nowhere the freight train can go to allow it to pass.

The DoT could fine the railroads each time they do that until they either run shorter trains or build the appropriate sidings, but they choose not to.