The frequently repeated claim that "freight removes trucks from the road" , like many simplifications, it is not always true. (It's not always false, either.) The Transportation Research Board commissioned a detailed question on the issue because so many entities looking at the issue were confused by the wild claims of rail supporters and -- like many simplistic claims -- the real answer is complicated to determine. Here's a quote a from "ASSESSING RAIL FREIGHT SOLUTIONS TO ROADWAY CONGESTION: FINAL REPORT" (p. 9)

"It is important to recognize that there are multiple market segments served by the
trucking industry, not all of which are competitive with rail. Local and regional
trucking accounts for the vast majority of truck movements in urban areas, and rail is
competitive for almost none of this traffic (high volume moves of sand & gravel, road
salt, coal, or oil products are the major exceptions). Rail and intermodal are options
for intercity traffic that is traveling several hundred miles or more. This traffic
includes small shipments that are commonly shipped in less-than-truckload (LTL)
amounts as well as truckload (TL) shipments. Rail/truck intermodal is an option for
both LTL and TL shipments if the service is reasonably fast, reliable and efficient
compared to the trucking option. Rail carload must have rates that are low enough to
offset the added logistics costs associated with the slower, less reliable service and the
requirement for larger shipment size.

There is no hard-and-fast distance that demarcates rail and trucking zones. Trucks
provide some transcontinental service, while rail provides some local and regional
services. However, the average rail shipment is over 500 miles, whereas the average
truck shipment is under 300 miles. The better the rail service in comparison to truck
service, the shorter the distances for which rail is competitive – and vice versa."

The conclusion I draw is that this question has not been answered for the region potentially served by the NCRA and should be carefully examined. And, I'll bet, the answer depends
on what one assumes about how whether freight makes it through the Eel River Canyon and whether it serves a container port in Humboldt Bay.

For example, freight service limited to Willits south does not appear to be economically viable, once future track repair/maintenance issues are considered. Yet, this is the one example of where freight service may remove trucks from the road -- even if it is a small number.
Throw in the Island Mtn Quarry, and suddenly you have a lot of freight cars and no substitution effect.

So, it matters what you assume about where the NCRA is really going.