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.