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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When a car runs on a road, its rubber wheels squish slightly. This squishing takes up energy, slowing the car down.
What’s more, if the road isn’t smooth it causes the car to bump up and down, what also takes up energy.
Steel wheels don’t squish much, and steel train tracks are durable enough that they stay smooth for a long time.
Bumps use up energy.
When you roll over a bump, you are lifting the entire carriage a tiny amount. That energy has to come from somewhere. You get some of it back on the other side of the bump but not much of it.
Air filled tyres are better than solid for the same reason. Going over a bump the tyre deforms a little but that loses a lot less energy than lifting the entire vehicle.
Rails are smooth, much smoother than roads (especially roads in the past). If you go over a bump in the road, you have to pull hard to lift the weight of the thing you are pulling up and over the bump. On smooth rails, there is much less pulling up and over bumps. Additionally, steel wheels on steel rails have very low rolling resistance. When the steel deforms (just a little) at the point where the wheel and rail are in contact, most of the defamation is purely elastic, so it does not use up energy in the process of rolling. Other materials, notably rubber, deform a lot at the contact patch when there is weight on them, and the nature of the material means this deformation takes up energy. As well, there are parts of the contact area where the wheel and surface slide over one another, with a high frictional force, so energy is used up in that process. These effects are generally termed “rolling resistance”. While people sometimes say that steel wheel on steel rail have low friction, really it is rolling resistance that is the more important effect.
A huge reduction in rolling resistance, mostly. The contact patch of a steel wheel on a steel rail is very small, and the two materials compress and conform to each other to a nearly negligible level. All of that equates to very little drag. It also explains why the wheels can lock up and slide easily under braking or hard acceleration.
Another example is riding a bicycle. It’s much easier to ride on a hard surface on a bike with hard, narrow tires, like a racing bike, as compared to a soft surface on a bike with wide, under inflated tires, like a mountain bike on a beach.
Friction and pathways.
Putting something “on rails” makes a determined pathway with navigable channels. By these two metrics alone rail travel is much smoother and faster than roads…
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