Why does a tram wear a little ponytail on its roof?

A tram burns no fuel and puffs no smoke; it stops, opens its doors and slides away almost silently. Its strength comes from two things: the leaning “ponytail” on the roof — the pantograph — and the two shiny steel rails under its feet.

The electricity arrives along one supply chain: the distant grid feeds a traction substation along the line, which conditions the voltage and sends it up to the overhead line; the pantograph slides along and drinks, feeding the tram’s motors. The current then completes a circuit: it leaves through the wheels into the rails and returns along the return path to the supply — out and back, one loop. The energy handed to the motors becomes motion and heat; the current itself is not a thing that gets “used up” — it flows around the loop. The rails don’t just steady the wheels; they double as the way back.

Why run on rails at all? A steel wheel on a steel rail touches over a patch the size of a fingernail (about 100 mm² for a laden wheel) — hard steel on hard steel barely deforms at all, so rolling is wonderfully easy. Steel-on-steel rolling resistance is around 0.002 against roughly 0.012 for a rubber tyre on asphalt: for the same tonne, the tyre works six times harder. Where does the effort go? Into the tyre’s own body: soft rubber flattens at the contact and springs back every turn, flexing and heating and wasting part of the push.

When cars queue in long lines, a tram with its own lane feels far less of the jam — though that is not true everywhere: in mixed traffic and at junctions the tram still obeys signals and dispatching, and in rush hour it waits too. At junctions, the points are a swinging blade of rail: tip it left and the wheels are guided onto the left track; tip it right, onto the right. The driver never chooses — the track chose beforehand.

No fuel means no exhaust, and the motors only hum. Cities are inviting trams back — rails through the flowerbeds, wires overhead — and the tram drifts through town carrying carload after carload, leaving the street a little quieter.

Detailed controls

1. Chase the current. In chapter two, three golden dots travel grid → traction substation → overhead line → pantograph → motor → wheels → rails → return (the dots sketch the direction of the current, not little lumps of electricity being consumed). The exploration keeps a “show the current loop” switch.
2. A fingernail of contact. Chapter three’s magnifier shows the tiny bright patch a steel wheel leaves on the railhead — beside a soft tyre that flattens and springs back every turn. The saved effort reads “×6” at the top.
3. Jam the street. Drag congestion to the top: the queue of cars grows and the car’s travel time multiplies while the tram’s number never moves.
4. Flip the points. Press left or right: the yellow blade swings in the plan-view card and two little trams run each branch.

Guess first: how does this demo tram get its power — a big battery in the car, or the wire overhead? This one is fed along the line and draws power as it runs, so it never stops to charge. Other trams use ground-level supply or on-board storage for stretches without overhead wires.

Many tram points only “act” when a divergence is actually needed: for the straight move the blade rests lightly against the straight stock rail and the wheels simply pass. Before a diverging move, the track sets the blade in advance. Modern trams also talk to traffic lights for signal priority — on a well-planned line this helps them dodge much of the congestion.

Rolling-resistance coefficients use 0.002 for the rail and 0.012 for the tyre (common literature ranges are 0.001–0.003 and 0.008–0.015). The contact patch follows Hertz point contact, area growing with load^(2/3); a 30 kN wheel load gives about 100 mm². The tyre’s cost is drawn as the carcass itself flattening and springing back, not as a dent pressed into the road. The supply is drawn as grid → traction substation → overhead line → vehicle → rails → return path, with polarity and stray-current protection simplified for teaching. Trams without overhead wires exist (ground-level supply or energy storage); this film covers the overhead-fed kind only.

The pantograph’s joints fold on purpose: the contact wire dips regularly between supports (the sag is drawn here), and the head keeps a nearly constant pressure while the arms stretch and shrink to follow. This helps it maintain contact within its designed speed range. Metros commonly use either overhead contact lines or a third rail beside the track; overhead lines can fit inside tunnels too. Compare the two in the metro.