First a bit of train history.
Trains carry a lot of heavy and bulky goods for long distances. Now almost all trains use diesel engines for propulsion. But in the past most trains were electric and driven by overhead wires, the train used no fuel. The electricity could be from any source including hydropower.
I was in Germany in 1969 and they used electric trains with old coal burning locomotives for supplemental power. The diesel locomotives were not that common. That may have changed since then.
The Los Angeles metro area had a very good electric mass transportation system up until the 1960s when the system was dismantled.
The system worked then; why can’t it work now?
Constraints
In his book, Gates lumps trains in with planes and ships, probably because they’re heavy transportation. Airplanes have severe weight restrictions on them. But trains do not have such restrictions, and ships to an even lesser extent. A train has to accelerate and decelerate but the weight of each freight car is heavy. The rolling resistance of the steel wheels is low so the locomotive can pull a lot of cars with several thousand horsepower.
Another limit on trains is they can go only where the train tracks are. This also helps because the tracks are far fewer than roads or highways. The trains in some cities were powered by electricity and there is no reason why they can’t be changed back. The costs are not excessive. One has to determine if it is more economical to run electric lines overhead or if the trains should be required to run on biodiesel. It’s possible that both of these may be used.
Train cars have wheel brakes that are controlled by air pressure. The locomotive controls the air pressure and so controls the brakes.
My idea is to use electricity to control the starting and stopping. The locomotive could use KERS (kinetic energy recovery system) to slow down the train. The freight cars would have motor-generators on the wheels and they would generate electricity to slow down the car. Then the recovered energy would be used to restart the stopped train.
The freight cars have roofs with an area big enough to mount solar panels. Some energy could be recovered and used for propulsion. But much of the energy could come from an overhead line.
The area that has the biggest potential for technological advancement is the automation of the train system. With a track that limits the train to where it can go, the main action that needs to be automated is the stopping, starting and speed of the train. The safety of everything around the train is also automated like it would be with a full self driving car. If the cars communicate with each other and the locomotives then the cars could be made to propel themselves. The system could be automated to where the cars would self assemble and travel to a remote point where they would split off to individual destinations.
Many of the semi-autonomous activities that would be needed are already being used in other vehicles. But the most important thing is to electrify the trains and eliminate fossil fuels.