Six ways to make a toothed rail, each the answer to a different problem.
Every rack railway uses the same principle, a pinion on the engine meshing with a rack on the track, but the rack itself has been built in very different ways. Pick one, turn it round with your finger or mouse, and watch the pinion roll along it. The running wheels turn at their own speed: on many mountain engines they are loose on the axles and only carry the weight.
Each system answered the problems of the one before. Marsh's round pins worked but gave an uneven push. Riggenbach's ladder was strong but costly, hard to make with every tooth exactly in pitch, and it packed with ice in winter. Abt's flat bars were milled several at a time so that the pitch was dead true, and either bar could carry the load alone if the other were damaged. Strub rolled his rack like an ordinary rail. Locher, facing a mountain twice as steep as anyone had tried, turned the teeth on their side so the pinion could not lift out at all.
When the Snowdon Mountain Railway chose its rack in the 1890s, its engineer Frank Oswell reported in favour of Abt for exactly these reasons: true pitch, bars replaceable one at a time, and no cheeks for the snow to fill.
The height of the pitch line above the rail top is chosen line by line, not by system. The Swiss rules (VöV RTE 29700) tabulate it for 22 Swiss lines. A few examples:
| Line | System | Pitch line above rail (mm) | Tooth tip (mm) |
|---|---|---|---|
| Rigi | Riggenbach | 0 | 18 |
| Brienz Rothorn, Monte Generoso, Gornergrat | Abt | 35 | 50–55 |
| Wengernalp, Schynige Platte | Riggenbach, Von Roll | 40 | 56 |
| Jungfraubahn | Strub | 55 | 70 |
| Pilatus | Locher | 74 (meshing line) | 94 |
| Martigny–Châtelard | Strub | 123 | 138 |
Where the rack sits high, the pinion clears the running rails and points can keep their rails whole. Where it sits low, as on the Rigi, rails and rack must both be cut and moved at every set of points.
Sources: Engineering (1896), the Snowdon engines and track; E. Strub, Schweizerische Bauzeitung (1893), the Wengernalpbahn; The Engineer (1886) and Engineering (1887), Pilatus; V. von Röll, Enzyklopädie des Eisenbahnwesens (1923), "Zahnbahnen"; E. Strub, US patent 600,324 (1898); VöV D RTE 29700, chapter 1.