Rack & Pinion Railway

An ordinary train grips the rail by friction alone — and that grip runs out around a 10% slope. A rack railway beats the limit with hardware: a toothed rack rail runs between the two running rails, and the locomotive drives a matching toothed wheel — the pinion, or cog — that meshes into it like a gear into a gear rack. Grip is now mechanical, not frictional, so the train climbs grades that would leave a plain adhesion train spinning in place.

How it works

The rack is a fixed toothed bar bolted along the centre of the track. On the powered vehicle, an axle-driven pinion lowers into engagement so its teeth interlock with the rack's. Because the mesh physically resists slipping, propulsion no longer depends on weight-on-rail friction. Cog lines routinely work 25%, 30%, even steeper, and the principle scales to gradients of 100% — a full 45° — in the extreme. Crucially, this train carries its own engine — steam, diesel, or electric — and pulls itself up under its own power. That is the sharp break from the cable family on this map: nothing hauls it from above; grip and motor both ride on board.

The rack systems

Several tooth designs emerged, trading cost, strength, and maximum grade:

• Riggenbach (1871) — a ladder-type rack: steel plates set like rungs between side channels. Rugged and the earliest, but heavy and expensive to build.
• Abt (1882) — two (or more) solid bars with vertical teeth, offset so the pinion is always meshing on at least one bar. Smoother and lighter; it became the most widely adopted system.
• Strub (1896) — a single rolled rail with teeth machined into the head, simpler to lay and maintain, with safety jaws gripping the rail foot.
• Locher (1889) — teeth cut into the sides of the rack, engaged by flanged pinions from both sides at once. This double-sided grip stops the cog from climbing out of the rack under extreme load, which is why it alone handles the steepest slopes.

Pure rack vs. rack-and-adhesion

Two operating patterns exist. A pure rack line lays the toothed rail along its entire length — the cog drives and holds the train top to bottom. A rack-and-adhesion line runs as an ordinary friction railway on gentle stretches and drops the rack in only on the steep pitches, so it runs faster and cheaper where it can. The mesh pays a second dividend going downhill: the same engagement that stops upward slipping becomes a positive, controlled brake on the descent, holding a loaded car back on a fierce grade without trusting friction that might let go.

Where it climbs

Rack railways are the workhorses of mountain tourism and summit access:

• Pilatus Railway (Switzerland) — Locher system, 48%, the steepest anywhere.
• Jungfraubahn (Switzerland) — Strub system, tunnelling up to Europe's highest railway station.
• Mount Washington Cog Railway (USA, 1868) — the first mountain-climbing cog railway in the world.
• Snowdon Mountain Railway (Wales) — Abt system, to the summit of the highest peak in Wales.

For the deeper question of exactly where friction gives out and teeth take over, see adhesion vs rack.