Keywords: railways, rolling stock, wayside monitoring, accidents, safety.

Recently published documents by the European Railway Agency have identified and classified railway accidents in the last twenty years across Europe. These documents are of fundamental importance to understanding the root causes of the most important disasters where many people were killed and many more were injured.

The question of whether these accidents could have been avoided by using wayside train measuring systems (WTMS) is certainly crucial to the development of the railway sector, which is open to railway undertakings; that operate a large variety of vehicles on many routes, often across national borders. The definition of an entity in charge of maintenance (ECM) is, in fact, only a part of the problem: as long as the defects of incipient failures can be detected in a workshop; but quite often they arise during service in a situation which is quite rare hence the risk is low, but potentially very harmful to railway personnel, passengers, and people living nearby (i.e. the danger is high).

WTMS, i.e. those systems that check all possibly hazardous conditions of a running train, are made of different families and technologies of sensors and data processing techniques. Fire, chemical, mechanical, geometrical, and electrical interactions of a vehicle with the environment may require completely different approaches, united by the fact that some features can only be observed during service and not in a workshop.

WTMS equipment is becoming widespread in many important railway networks with the goal to both avoid accidents as much as possible, and to give railway undertakings the opportunity to optimize maintenance and, therefore, to save money when conducting their business. Safety issues must (or should be) addressed immediately by interfacing WTMS with the signalling equipment, i.e. when a safety-related issue is discovered the signals should be set to red and the train should be immediately stopped. Signaling issues and the related safety integrity levels (SILs) requested by the European standard may be very demanding in terms of WTMS architecture. However, maintenance works operate on typically slowly varying parameters.

This review paper describes the current wayside train monitoring system technology with particular reference to devices and technologies which are readily available on the market. After a first general description of the interaction of a train with the environment and the analysis of officially available data on accidents, a distinction was made between safety critical and maintenance related characteristics exhibited by a train during a pass-by. The rest of the paper is devoted to the analysis of safety related issues which, in some terms, require that WTMS systems should be implemented throughout Europe, while the analysis of those parameters that are related only, or mainly, to maintenance, which suggest that WTMS systems could be implemented, is not addressed.

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