In the field of secondary suspensions, besides wide-spread use of tilting there have been numerous attempts and research efforts. ADtranz in Sweden has looked into "semi-active" electronically-controlled lateral dampers for the X2000 tilting train. Similar efforts have been done at ABB, Alstom and others. In Japan hydraulic and pneumatic actuators have been tested on the WIN350 train.

Implementation of active primary suspensions is still further away. Some research has been conducted in the Netherlands for the Rotterdam tram as well as in Germany and Austria. The Copenhagen S-trains with (LINK!) KERFs do not use active suspensions in the strict sense used here, but represent an important development in this direction.

An international research consortium "Mechatronic technologies for trains of the future" (funded by the European Commission) is currently studying the potential of these and other mechatronic developments for future train design.

• Weight reduction
• Reduced mechanical complexity

Reduction of wear and tear

Less wear on wheels and track through improved curving capability

Weight reduction

Active steering makes two-axle bogieless vehicles feasible, offering advantages such as

• reduced mechanical complexity (which of course is paid for by a higher degree of electronic control complexity)
• reduced vehicle weight
• reduced traction/braking requirements
• reduced energy consumption
• reduced track damage

Extra functionalities

A mechatronic approach may bring added value of a different kind, e.g. the integration of the suspension, guidance, drive and braking sub-systems which today are designed and controlled separately. The use of differential torque control to achieve active steering or guidance is a step towards higher degrees of system integration. In addition, condition and fault diagnosis for maintenance purposes could be provided at low extra cost, making use of the sensors fitted for mechatronic control.

Technological inertia

Primary suspensions are an extremely fundamental and safety-critical sub-system of a railway vehicle. Therefore it will not be easy to replace the well-known and mature bogie technology by innovative technologies.

Safety and reliability

Primary suspensions are still confronted with a number of safety problems. More research is needed.

Actively controlled running gear meets very high acceptance barriers due to doubts about safety and reliability. Success factors are:

Thorough risk assessment

Make a thorough risk assessment of the whole vehicle rather than running gear alone.

Communication strategy

In order to overcome scepticism, a convincing communication strategy is essential. The focus should be put on successful use of active controlled mechanical elements in other industries:

• automotive: Anti-lock braking system (ABS), Active stability control (ASC)
• aerospace: fly-by-wire etc.

But also in railways, tilting trains are an example of successful introduction of active control elements and are seen by experts as the "tip of an iceberg" (Goodall, Kortüm 2000) that could facilitate introduction of mechatronic solutions in other fields.