Steel on steel, barely gripping
How Can One Engine Pull a Two-Kilometre Train?
A train rolls so easily that one engine can drag ten thousand tonnes — and grips so poorly that stopping takes a mile.
-
Step 1 of 5
A rolling wheel is fighting the squash, not the friction.
A rubber tyre flattens where it meets the road, and unflattens behind. That flexing costs energy on every turn, all day long.
-
Step 2 of 5
Steel barely squashes at all.
A steel wheel on a steel rail touches over an area the size of a coin and hardly deforms. Rolling a wagon takes roughly a tenth of what the road version takes.
-
Step 3 of 5
So a modest engine can move an absurd load.
Pulling ten thousand tonnes needs only enough force to overcome that small rolling drag, plus any slope. On the flat, it is far less than it sounds.
-
Step 4 of 5
The same smoothness is why it cannot get going or stop quickly.
Grip comes from the same contact that is barely touching. A wheel that spins uselessly cannot push the train, and brakes that lock the wheels just slide.
-
Step 5 of 5
Which is why a train needs a mile of warning.
A car stops in a few dozen metres. A loaded freight train needs well over a kilometre, and that distance is why level crossings and signals exist at all.
Sand is carried on board and dropped in front of the driving wheels when the rail is greasy — a deliberate way of spoiling the smoothness that makes it all work.
The short version
Steel wheels on steel rails barely deform, so rolling costs a tenth of what a road costs — which is why one engine pulls so much, and why it cannot stop.
Try it yourself
Push a loaded shopping trolley on smooth floor, then on carpet. The carpet squashes under the wheels, and that squashing is the whole difference.