Why does putting a carriage on rails make it much easier to pull? As in, how were the first trams such an improvement from omnibuses when the same weight was still being pulled?
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Great question.
Part of it is materials: metal rimmed wheels roll better. And metal tracks are better to roll on.
But part of it is also infrastructural: building a rail track involves carefully laying down a guide system that covers every meter from point A to point B, and has to be smoothly aligned and joined and so on. *Anything* you do with that degree of precision is going to offer a considerable improvement in terms of energy efficiency.
On the second point, consider the example of an inflated rubber tired automobile on an asphalt surface. Kind of the opposite of a steel wheel on a steel track, in a lot of ways, right? But pushing that automobile is surprisingly easy, at least as long as the grade is close to level.
Try pushing that same automobile across an open meadow. Sure, you’ll probably get a certain amount of distance. If you’re dedicated, and maybe have some friends to help, you might be able to keep it going for a while. But it’s a huge effort to go anywhere — you are constantly struggling against irregular terrain. And sooner or later you are going to hit some dip or shallow depression that you will really struggle to get out of.
By contrast, if you start pushing a car along a decent paved road, a single fit person can keep it up literally for miles.
So, historically, I think to some extent the advantage of rail-guided vehicles was simply that for the first time someone had really laid out a road with a very high degree of care and engineering.
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