Trucks powered by internal combustion engines? It's not an all-or-nothing thing; many products even today have a rail segment somewhere in their delivery to the consumer. Even so, for rail to grow much it has to take business from trucking.

There wasn't much in roads or trucks for long distance travel in the early 1900s. I believe it took roughly 30 days to do the trip across the country on the Lincoln Highway as opposed to 7 days via the train.

I would expect if you look at the volume of freight shipped via rail vs trucks it wouldn't be until the 1950s and the creation of the interstate system that it really took off. Containerization in the 50s and 60s would be another big boon to trucking.