Rack Railways

The rail system is the most energy-efficient means of transport and will probably always remain so. The reason for this is the low rolling resistance. The disadvantage, however, is that with the so-called adhesion rail, the braking distance is considerably longer than with road traffic, for example, and the drive power should not exceed the low static friction in the wheel-rail contact, because then the wheels would spin.

While this only leads to slower acceleration on level ground compared to road-bound traffic, this becomes a traction problem as the incline of the track increases, even at comparatively low values. In the case of mainline railways, this leads to routes being designed in such a way that gradients of over 30‰ are the exception. Freight trains in particular, with only one locomotive and many heavy wagons, quickly reach the limits of what is physically possible. If the speed uphill is mainly limited by the combination of drive power and static friction, the maximum speed downhill is also limited due to the longer braking distance.

Passenger multiple units with distributed drives have better power transmission, similar to an all-wheel drive vehicle on the road, so that they can travel on steeper gradients. However, from a gradient of around 70‰, the maximum is also reached in this case. In the mountains, the topography can be so demanding that even multiple traction is no longer sufficient. There are therefore two options for the routing: Either the maximum gradient can be kept low by artificially lengthening the line by means of loops and helical tunnels (e.g. Sauschwänzle railway, Black Forest railway, Gotthard mountain line, Bernina railway), or a rack is mounted in the centre of the track, in which a cogwheel mounted in the rolling stock engages in a form-fitting manner and thus supports the power transmission or even takes over completely.

Oldest cog railway in Europe: The Rigi Railway above Lake Lucerne, Switzerland
© Patrick Braess

In addition to adhesion railways, there are pure rack-and-pinion railways where the drive is always via the cogwheels and the wheels are not driven (Gornergratbahn, Pilatus-Bahnen, Brienzer Rothorn Bahn, etc.), and there are mixed adhesion and rack-and-pinion railways (Matterhorn Gotthard Bahn, Zentralbahn, Bergbahn Rheineck – Walzenhausen, etc.) where both types of drive are used. The advantage is that the infrastructure in areas that do not require a cogwheel drive can be travelled on with the adhesion drive and the additional rack only needs to be installed on steeper gradients. Routing can be much more direct, as no loops or tunnels are required. The disadvantage is that the vehicles have a much more complicated design and are considerably more expensive due to the small quantities. The travelling speed uphill is also limited to 40 km/h, and even lower on the descent due to the braking distance. To reduce journey times, for example, a new tunnel is being built between the towns of Täsch and Zermatt on the Matterhorn Gotthard Railway, in which the rack rail can be dispensed with thanks to a more constant inclination. This reduces the journey time significantly.

With mixed adhesion and rack-and-pinion tracks, the train must enter the rack at the beginning of each rack section, i.e. the rack must be synchronised with the tooth pitch of the rack. When entering the rack, the train must be slowed down to 10 km/h and can only be accelerated again to a maximum of 40 km/h (uphill) in the rack section. Especially with longer train compositions, this leads to a loss of travel time of up to one minute per entry. For this reason, crossings on single-track lines are often equipped with racks throughout, despite the preferably low gradient, even if this involves considerably higher investments in the infrastructure. The exit from a rack and pinion section can take place at standard speed.

Historically, three different main groups of gear drive systems have been developed:

1. Riggenbach, Strub, von Roll
All three rack and pinion systems can be used by the same vehicles. The tooth spacing is 100 mm. Riggenbach profiles are usually 3 m long and therefore have 30 teeth. These profiles, also known as ladder racks, cannot be adapted to the radii required on site due to their rigidity. Therefore, in addition to straight profiles, there are also curved profiles in two different radii as standard. For other radii, any radius can be created by combining the two different curved profiles, taking advantage of the clearance at the joint. Riggenbach profiles are no longer produced today due to the very labour-intensive production method. However, the teeth can be recharged by welding on multiple times, thus extending their service life.

Riggenbach rack rail at the Drachenfelsbahn in Königswinter, Germany
© Patrick Braess

Strub profiles are made from rolled profiles. The current TN 70 vignole profile (with a mass of 70 kg/m) is based on an SBB I(46E1) rail profile with a slightly narrower foot and a larger head. The teeth are cut out of the head individually. A major advantage is the position-independent fastening on the sleepers with classic rail fastenings. This means that the construction can be carried out in the same way as with a classic adhesive superstructure and the rack can be inserted and fastened after the final tamping.

Von Roll profiles are made from solid profiles, usually with a width of 60 or 80 mm. Based on the specified fixing points, sleepers and racks are installed first, followed by the rails.

Strub profiles and von Roll profiles can be aluminothermically welded after the installation is complete. This guarantees the correct tooth spacing at the joint in the long term.

Strub TN70 rack rail at the Manitou Pike’s Peak Cog Railway in Colorado, USA
© Patrick Braess

Von Roll rack rail at Zentralbahn, Central Switzerland
© Patrick Braess

2. Abt (2 and 3-lamellar)
The Abt system consists of two or even three profiles, each half offset from each other. The tooth spacing is larger than with the other systems (120 mm). The advantage is that the profiles are always installed offset, so there are no full joints. Due to the low thickness of the profiles, they can be fitted or rotated much more easily by hand, which makes maintenance easier. In the curve, the outer profile should theoretically be longer. With short profiles, the difference in length is equalized at the joint. Recently, however, longer profiles (9 m) have been used more frequently, so equalising profiles with a pitch error (> 120 mm) have to be installed in the curve.

Abt rack at the Cremallera de Montserrat, Spain

3. Locher
With the Locher system, the teeth are horizontal instead of vertical. This system is only used on the Pilatus rack railway. The advantage is the significantly greater inclination of up to 480‰. All other systems have a maximum inclination of 250‰.

Only a few of the approx. 40 rack railways worldwide are compatible with each other, even if only the superstructure is considered and the clearance gauge, current system, etc. are ignored. In addition to the different rack and pinion systems and the different track gauges, the height of the rack head to the rail head also varies. Adapting to a different system requires the parallel conversion of the entire rolling stock and infrastructure, which usually fails due to excessive costs and time. This means that the various rack railways will retain their respective uniqueness for the foreseeable future. Rolling stock orders will continue to be customised in the future.

Since Switzerland has the highest number of rack railways in the world and almost all rolling stock is manufactured in Switzerland, the regulations there are the most comprehensive, e.g. D RTE (Railway Technology Regulations) 29700 System Technology for Rack Railways.

_{In friendly cooperation with Tensol Rail SA}