It is my understanding that President Nixon signed the NRPC bill mostly to guarantee the continuation of passenger rail service in the Northeast U.S. in the wake of the Penn Central bankruptcy. Recall that PC operated over 20 % of ALL U.S. passenger trains at the time. As well, many U.S. Railroad CEO's had seen the future of their passenger trains without the recently departed U.S. Post Office Department Mail contracts. A non - existant future at that. Several CEO's argued successfully that since the Post Office Department, an agency of the U.S. Federal Government, had " created " the passenger train problem, that the railroads were due some form of relief from that same federal government. ( Never mind that passenger trains, w/o mail & express shipments, have NEVER made a net profit, in the U.S. or elsewhere. )
Yes, the SDP-40F was envisioned with a possible conversion to freight hauling in its' future. What was not envisioned was the U.S. Oil Crunches of 1973 & 1975, that saw passengers return to the rails.