Rack Railways
Aim - To describe the use and operation of rack railways.
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Index
Introduction
As early 19th-century mechanical designers started to build railway lines up to geographical high elevations it was determined that an adhesion type locomotive could not haul loads up steep gradients. Typically where grades were too steep (more than approximately 4% or 1 in 25) rack (cog-and-rack) railways were used as the preffered method of construction in order to avoid significant engineering works in building loops and/or switchbacks to lower the ruling (maximum) gradient. Thus in mountain regions, rack railways became predominant.
In a typical cog-and-rack railway, a toothed rack is centered between the ordinary running rails of what would otherwise be ordinary railroad track. The teeth in the rack were engaged by a cog or cogs attached to one or more of the locomotive's drive axles, providing extra traction that helped move the train.
Rack Railway Systems
There were many different rack railway systems developed over the years (Fig. 3), including the first, the Blenkinsop (1812), the Fell (1860s), the Marsh (1861), the Riggenbach (1871), the Abt (1882), the Agudio (1884), the Locher (1889), the Strub (1896), the Morgan (1900) and the Lamella/Von Roll. Each rack system had their own tooth profile, and each design had their own unique advantages and disadvantages.
The Riggenbach and Abt rack system seem to have been the most popular systems, and hence a lot of these were implemented.
Rack Locomotives
The following description has been translated from the 1903 reference document no 1 below.
Locomotives for rack railways can be classified according to the arrangement of the drive mechanisms as follows.
Locomotives for pure rack railways - these locomotives are propelled solely by the pinion on the rack, and thus otherwise only possess non driven adhesion support wheels. In these machines, the pinion drive wheel is generally driven by a faster-running crankshaft via a gear ratio.
The system of the pure rack locomotive, first used by Riggenbach on the Rigi Railway, is suitable for railways that ascend to high points with steep, relatively constant gradients and have to haul comparatively light loads. Coupling the track wheels would be of no use with these machines, since the adhesion of these wheels could only provide a small fraction of the required tractive force. The drive of the cogwheel is usually carried out by means of a fast-running reduction gear. The purpose of the reduction gear is to increase the boiler output to the level achievable when the steam engine is running at high speed and to reduce the dimensions of the drive mechanism.
Locomotives for mixed operation - locomotives which are capable of traveling on both pure adhesion sections and rack railway sections. Here, a further distinction can be made:
- Locomotives whose road wheels are fully or partially coupled to the gear axle by cranks and rods. The gear is driven as described in I. These machines can therefore also move on purely adhesion tracks, but only at low speeds due to the wheel ratio.
- Locomotives whose gear axles are connected to the similarly driven adhesion axles by cranks and rods, which can therefore travel on smooth tracks at higher speeds, like ordinary locomotives.
- Locomotives which have completely independent steam engines for the rack and pinion drive and the adhesion drive, and can therefore also run on smooth tracks like other locomotives.
Pure Rack Locomotive
The system of the pure rack locomotive, first used by Riggenbach on the Rigi Railway, is suitable for railways that ascend to high points with steep, relatively constant gradients and have to haul comparatively light loads. Coupling the traction wheels would be of no use with these machines, since the adhesion of these wheels could only provide a small fraction of the required tractive force.
The drive of the gear is usually carried out by means of a high-speed reduction gear. The purpose of the reduction gear is to increase the boiler output to the level achievable when the steam engine is running at high speed and to reduce the dimensions of the engine. The construction of these locomotives will not be discussed in more detail below; rather, only the locomotives of the second type, those for mixed operation, will be examined in more detail.
Rack Locomotives for Mixed Operation
The system of rack locomotives for mixed operation is used for railways where gradients of varying sizes alternate with level sections, so that the locomotive must be able to operate both as a rack locomotive and as a conventional adhesion locomotive.
Type 1 - The locomotives designed by Riggenbach, whose rack axles are driven and whose suspension wheels are coupled to these axles, were the first locomotives for mixed operation ever built.
The drive mechanism is arranged with two cylinders, usually mounted on top, drive a dummy axle, through which the motion is transmitted to the rack axle by means of a gear reduction. The rack can be driven either indirectly or directly.
In the case of indirect drive, two small gear wheels are mounted on the dummy axle, which engage with two larger ones located on the rack axle. Between these larger wheels, keyed to the same axle, sits the actual rack.
With direct drive, the dummy axle has only one small gear which engages directly with the rack and pinion without an intermediate link.
This latter arrangement saves three gears, thus reducing both manufacturing and maintenance costs. The driving of the axles is achieved either
- from the dummy axle (Ostermundingen) or
- from the rack and pinion axle, either by a) coupling rods or b) by gears
The disadvantages of the first type are the following:
- Due to the rack-and-pinion transmission, the locomotives can only travel slightly or not at all faster on level track than on rack railways.
- In those locomotives whose axles are driven by coupling rods from the jack axle, the natural adhesion of the axles cannot be utilized on rack railways, as they can only act as running gears due to their circumferential speed differing from that of the driven gear.
- If adhesion is used on the rack sections (when the axles are driven by means of coupling rods from the rack axle), the difficulty arises of keeping the diameter of the adhesion drive wheels approximately equal to that of the rack drive wheels. The former are therefore manufactured slightly larger than the later when new, so that they slide somewhat on the rack sections. This sliding causes significant wear on the wheels and rails and consumes a great deal of tractive force.
- The large number of gear wheels, which also run either driving or idle when traveling on purely adhesion sections, results in high maintenance costs.
The advantage of wheel transmissions, as already mentioned with regard to pure rack-and-pinion locomotives, is that the boiler output is increased by the faster running of the steam engine.
Type 2 - In the second type of locomotives for mixed operation, the aforementioned disadvantages have been partially avoided. The drive in these machines no longer comes from a separate dummy axle, but directly from the cylinders to the rack-and-pinion axle. This achieves, above all, the great advantage that the locomotives can travel at higher speeds on smooth track. Furthermore, all the disadvantages arising from the rack-and-pinion drive are eliminated (see above items 1, 2 and 4), and with them, however, also the advantage of better steam generation. The disadvantage mentioned under 3, regarding the slippage of the adhesion wheels and the resulting wear, persists and is a significant obstacle to using this type of locomotive for greater tractive forces, i.e., large, high-performance machines. For small locomotives, however, which are intended for material handling or shunting operations at railway stations, or for other reasons requiring the simplest possible design, this type is well-suited.
Type 3 - The disadvantages of the first designs were completely eliminated by the type of drive invented by Roman Abt, while all the advantages of these systems remained. Abt employed two completely separate and independent steam engines, one for adhesion operation, the other for rack and pinion operation. On smooth sections, the former operates alone, while the latter remains stationary. On the rack and pinion, both engines operate simultaneously, so that the entire tractive force available at the adhesion wheels is always utilized.
The driving wheels of the rack-and-pinion engine are built so small that even on rack-and-pinion sections a sufficient rotational speed for good steam generation is achieved, although the adhesion engine operates slowly.
These engines can therefore be built like ordinary mainline locomotives for any speed and any tractive effort and are capable of meeting all the demands of heavy traffic.
The drive mechanisms of these locomotives are arranged as follows:
- Adhesion drive - The adhesion driving wheels, all or only partially coupled, are driven by two external cylinders in the same way as in ordinary locomotives. There is no connection whatsoever with the rack-and-pinion drive.
- Rack-and-pinion drive - The gears are usually driven by two internal cylinders, either by lever drive, gear reduction, or a direct connection via a connecting rod. Lever drive was used in the first Abt-type locomotives.
This arrangement allows for horizontal mounting of the internal cylinders, but it increases weight and has no effect on steam generation.
Lever drive is therefore no longer used by European factories. Only in America are locomotives with this type of drive still being built; in these American locomotives, almost all cylinders are arranged vertically, making lever drive the simplest, if not the only possible, option.
The drive via a gear reduction is adopted by classes 1 and 2, but is only implemented sporadically in class 3.
Since Abt's locomotives are almost exclusively designed for high power outputs and tractive effort, the stress on the small gear is high; it wears down quickly and then causes chugging, a jerky running of the locomotive on downhill runs, and malfunctions in the operation of the valve gear and the air brake.
While this type of drive has the aforementioned advantage of generating higher boiler output, the latter can also be increased sufficiently with direct drive and adequately small gear wheels.
Since Abt's locomotives are almost exclusively built for high power outputs and tractive effort, the stress on the small gear is considerable; it wears out quickly and then causes problems with the operation of the valve gear and the air brake. The direct drive via a connecting rod is by far the simplest and safest and is used exclusively today. The gear cylinders must be arranged at an angle to allow the drive mechanism to clear the front axle, but this inclination can be so slight that the errors arising in the valve and piston movement are negligible. In Abt's machines, the gear wheels are mounted in a special frame, which rests on the axles of the adhesion wheels. The driving force is therefore first transmitted to the supporting axles and through them to the frame, resulting in considerable friction and high stress on these axles and bearings. As a result of this frame mounting, the gear wheels do not participate in the machine's suspension, and suspension springs of any desired stiffness can be used, which is important for driving on flat stretches at higher speeds. In locomotives of the earlier design, the springs had to be quite rigid in order to keep the height of the gear wheels, necessary for proper engagement, as unchanged as possible. For this reason, achieving higher speeds was therefore also impossible with these later machines.
In Abt's machines, the height of the gears changes only due to the wear of the wheel grooves of the support bearings. To restore the correct engagement, the gears can be raised by inserting shims into their bearings.
The gear frames themselves are now conveniently manufactured from cast steel, a design which, compared to sheet metal frames, has the advantage of low cost and ease of manufacture.
Two types of gears are used: one for gears intended to operate in a Riggenbach-type rack, and the other for gears intended for the Abt rack.
The Riggenbach rack consists essentially of two parallel angle irons or a U-shaped iron, between whose vertical legs the teeth are riveted as rungs of a suitable cross-sectional shape. Usually, two coupled gears are used to distribute the pressure on two sections and reduce wear on the gears.
The Abt rack consists of two or three flat iron bars placed vertically next to each other, from which the tooth profiles are cut. The teeth of the flat iron bars are offset by half or one-third of the pitch; each gear is composed of two discs, which are offset from each other in the same way as the parts of the rack.
Abt constructs the gears in such a way that the tooth circle and the wheel discs are not made from a single piece, but are connected to them by springs. The springs are dimensioned so that they only become effective when the respective disc is significantly overloaded due to unequal spacing of the rack. The springs then give way, and no increased stress is placed on the individual teeth. This arrangement gives the machine a very smooth running. However, more recently (on the Jungfrau Railway), the teeth of the flat iron plates are cut so precisely using special machines that Abt's design of gears can be abandoned, as a sufficiently smooth running of the locomotive is also achieved with conventional gears.
The gears made from a single piece of wood are, firstly, considerably easier to manufacture and, secondly, offer a high degree of operational reliability, which seems very questionable with Abt's gears.
The spacing of the gear axes in both Riggenbach and Abt's designs deviates from a whole multiple of the pitch by 1/2, 1/4, and 1/6 of the pitch for single-, double-, and triple-rail racks.
This arrangement produces an exceptionally smooth running of the machine, specifically with the Abt rack and pinion in a different way than with the Riggenbach rack, since, for example, with a pitch of 120 mm, 2 coupled gears, and a bipartite rack, a new engagement occurs every 30 mm, while with the Riggenbach rack, under the same conditions, a new engagement only occurs every 60 mm. With the Riggenbach rack, the gears in the rack must have 15-25 mm of lateral play after each step, while with the Abt rack, the gear teeth must be 25-30 mm further back than the Toothed rails.
This results in the disadvantage for the Abt's gears, that they only utilize the width of the lamella, whereas with the Riggenhach gears, the entire tooth surface is in contact.
Only involute toothing is suitable as a tooth shape, since only with this is the concavity of the gears caused by the wear of the wheel rims permissible.
This tooth shape has the advantage that the teeth on the rod have straight flanks.
The brakes are of particular importance in all gear systems, since the safety of the operation depends primarily and in particular on their effectiveness as well as on the adhesion forces.
Typically, every rack locomotive is equipped for mixed operation with three different brakes:
- Block brake - is constructed like that of ordinary locomotives and is used only for stopping during shunting movements.
- Brake for the gears - On both sides of each gear, grooved brake discs are bolted to it, upon which wedge-shaped brake shoes act. The brake shoes are usually arranged in a large number one behind the other on a steel band, so that the brake acts similarly to a band brake. This wedge-groove brake should generally not be used as a primary brake but only as a reserve brake, since continuous use on the side would cause significant wear and heat buildup.
- Air counter-pressure brake (sometimes called Riggenbach Brake) - is used for continuous operation, i.e., primarily for regulating speed on continuous downhill gradients. With the control lever closed, the operator returns the lever fully to the reverse, so that the pistons draw in air and it dries into the valve chest. The pressure in the valve chest, which forms the back pressure on the pistons, is easily regulated by a relief valve operated by the operator, so that the intended speed is maintained. Since this pressure can occasionally rise above that in the boiler, the steam regulator must be designed so that it cannot open automatically towards the boiler. A gauze filled with small stones, shavings, or sieves must be attached to the exhaust pipe of the exhaust valve as a silencer to eliminate the loud noise of the escaping air. To ensure that the pistons draw in clean air and not the gases from the smokebox with its impurities, the smokebox is closed off from the cylinders by a valve, and at the same time a connection to the outside air is created. To prevent excessive heating, some water is injected into the valve chests, which, mixed with the air, escapes from the silencer as steam.
Since Abt's locomotives have two separate steam engines, they are also equipped with two air brakes. Furthermore, these locomotives, being almost without exception built for high performance, are also equipped with an automatic continuous brake for braking the train, so that there are five brakes for the locomotive and train as a whole, thus ensuring a high degree of safety during operation.
The dimensions and proportions of the rack locomotives exhibit remarkable differences from those typical of ordinary locomotives in various respects.
In particular, the stress on the boiler heating surface, which, despite the locomotive's low operating speed at maximum effort, often exhibits high values. These values are permissible and are actually achieved by the boiler, because, due to the exhaust from four cylinders in Abt's engines, and due to the faster running of the steam engine through the use of gear reductions in Riggenbach's engines, the fanning of the flame is both strong and even.
Large flues are also frequently used successfully for the same reason.
Examples of Rack Railways
The following selection of information have been extracted from various documents in the Useful Lnks section below.
Route |
Build date |
Gauge (m) |
Length (km) |
Min Curve (m) |
Max. Grade (%) |
Construction |
Train Weight (tons -us) |
Rack Type |
|---|---|---|---|---|---|---|---|---|
Sumatra, Asia |
1891-4 |
1000 |
30 |
150 |
12 |
Rack / Adhesion |
|
Riggenbach |
Hollenthal, Black Forest |
1887 |
1435 |
35 |
|
5.3 |
Rack / Adhesion |
|
Riggenbach |
Mt. Washington, New Hampshire |
1868 |
1435 |
5.3 |
|
37 |
Rack |
|
Marsh |
Diakophto, Kalavryta, Greece |
1890-1 |
750 |
23 |
A - 30, R - 50 |
A - 3.5, R - 14.5 |
Rack / Adhesion |
148 |
Abt |
Hartz Ry |
1884-5 |
1435 |
30.5 |
A - 180, R -200 |
A - 2, R - 6 |
Rack / Adhesion |
17.5 |
Abt |
Montserrat Ry |
1891-2 |
1000 |
8 |
80 |
A - ?, R - 15 |
Rack / Adhesion |
10 |
Abt |
Manitou, Pikes Peak, Colorado |
1890-2 |
1435 |
14.2 |
109 |
R - 25 |
Rack |
20 |
Abt |
Mt. Generoso |
1889-90 |
800 |
9 |
60 |
R - 22 |
Rack |
11 |
Abt |
Transandino, Chile |
1890-2 |
1000 |
50 |
A - 115, R - 200 |
A - 2.5, R - 8 |
Rack / Adhesion |
66 |
Abt |
Useful Links
Critical description of the rack locomotives built to date for mixed operation by Arthur Werner (German text) (Includes some detail information on rack locomotives)
Locomotive Engineers Pocket Book - 1928 Read from pg 119
Transactions of the American Society of Mechanical Engineers - 1893 Read from pg 708