The Hidden Reference

I was writing my book last night. This current chapter discusses how supplies were moved after they were unloaded from Liberty Ships in the port of Antwerp. I remember reading somewhere that the Allies preferred to send their supplies by train where possible. I think this was because it was more fuel efficient. What I need to do is find that short passage in one of the many reference books I collected. Or was it in one of the books I returned to the library? Is it in my notes from interviewing veterans? I don't know. Feeling rather frustrated, I was inspired to draw a quick comic about the situation. Maybe the source will reveal itself later.