The concept of single-axle bogies is old and has been realised in several trains including the Spanish Talgo. Since bogies take a substantial share of overall train mass, single-axle bogies make an appreciable contribution to mass reduction.

Single-axle bogies have potential in both passenger and freight service. In freight operation, the challenges arising are however very different from those in passenger operation. This evaluation is focussed on single-axle concepts for passenger operation.

KERFs

One of the challenges of single-axle bogies has always been the optimisation of wheelset orientation in order ot reduce wear and noise. In the 1990s this problem was solved in a satisfactory manner by the Alstom development of KERFs ("Kurvengesteuerte Einzelradsatz-Fahrwerke"), curve-steered bogies with self-steering wheelset. KERFs were realised in the Copenhagen suburban trains and in the Alstom LIREX experimental train.

Although there is a variety of concepts for single-axle bogies, the following focusses on KERFs, presently being one of the most promising realisations of single-axle running gear.

Train concept

KERFs can only be realised in articulated trains. In the case of the Copenhagne suburban trains, the full train rests on 10 KERF wheelsets, compared to 16 for the previous generation of urban trains.

Figure 1: Composition of Copenhagen suburban train with KERFs

Source: IZT

Steering concept of KERF

The main innovation of the KERF concept lies in the steering control. Passive hydraulic steering devices orient the wheelsets according to the angle between two coaches. The flexibility of the wheelsets however allows for an improvement of the steering input by self-steering.

The new Copenhagen S-trains show that single-axle running gear of the KERF type are a mature technology. There is however still considerable potential for further technological development, especially as far as the self-steering concept is concerned. There are many possible lanes of future development going far beyond the KERF concept, especially in the field of mechatronically controlled running gear.

Mass reduction

Besides reduced energy consumption, weight reduction leads to higher acceleration rates for given traction power (or less traction power to be installed for a given acceleration rate).

Technological maturity

KERFs being a relatively new technology with only one train type running in regular service (Copenhagen S-train), there is still some scepticism about the reliability of the concept.

Single-axle bogies improve energy efficiency due to mass reduction. In order to estimate the mass reduction potential through KERFs, the new Copenhagen S-trains (cf. Description) may be taken as an example. They consist of 8 cars resting on 10 KERFs and have the same length as the old train sets composed of 4 cars resting on 8 conventional bogies. One KERF weighs about 50-60% of the mass of a conventional bogie (cf. Lenhard 2000). So the total mass of the KERFs along the train is about 62,5% - 75% of the bogie mass in the old trains.

Taking into account that in conventional EMUs, bogies (motor and trailer bogies) account for 30-40% of the total train weight, the weight reduction to be achieved with single-axle running gear is in the order of 7 - 14%.

The following elasticity table shows that the effect on overall energy effciency is 1-3 % in main line and high-speed transport and up to 9 % in local and regional transport.