The secret is in the cost of the interline through zone freight rate. That rate would be from the shipper location, maybe Chicago or Pennsylvania area, to the destination of Farmington, including the transfer per gage change. If the pipe was unloaded at Alamosa, the rate would be the same and any trucking movement would be expensive added cost to destination. The only way Rio Grande Motorway was used was at added cost to foster the abandonment of the railroad and removal of the low railroad freight rates. This was the era of Regulated Freight Rates, and they were built on past practice. It could never happen today with the railroad demanding what ever rate they want, and that would eliminate anything that did not have great value and many times over profit.
All of this history in included in the former ICC library which was loaded up into a giant herd of tractor trailer trucks and driven to Colorado Springs for CS College but they have never been opened to attempt to open to the public I understand. That job would be a giant adventure, and the man or woman power is not there to do that job. I think the hundred of trailers are still sealed up tight.