# Development and Testing of a Railway Bridge Weigh-in-Motion System
## 1 Introduction
Disadvantages of railway bridges are that:
* The mass of a train often represents a larger proportion of the mass of the bridge--this can result in changes in the dynamic behaviour of the system. Some of the more sophisticated B-WIM algorithms use the dynamic equations of motion to solve for the dynamic forces applied by axles to bridges [30; 31]. However, even this advanced method makes the assumption of a moving \"force\" on the bridge and neglects the dynamic interaction of the vehicle mass with the bridge. This assumption is generally reasonable for road bridges, where the mass of the vehicle is typically less than about 4% of the mass of the bridge. For railway bridges, the mass of the train may be 10% or more of the bridge mass. The influence of the large mass of a train on the dynamic behaviour of the system may cause inaccuracies in the calculated weights using standard B-WIM methods. It may be necessary to develop new, more sophisticated algorithms that allow for the influence of the train mass/bridge mass dynamic interaction. * Trains have many more axles than do trucks--trains consist of numerous axles that are generally in groups of two or three. Many closely spaced axles can lead to ill-conditioning of the equations used in conventional B-WIM systems. * Axle detection may be more difficult for railway bridges--train axles can easily be identified by instrumenting the rails. However, it is not always feasible to instrument the rail, specifically on busy lines where rail closure is not an option, or where the use of electrical resistance gauges is infeasible on electrified systems. Where the rail cannot be instrumented, it may be difficult to identify individual axles within groups, especially for ballasted tracks where the axle forces are distributed through the sleepers and the ballast and measured signals do not show peaks for individual axles. * In the light of the difficult and possibly erroneous axle detection, a general algorithm for rolling stock identification would be very complex. The issue is further complicated by the presence of Jacobs bogies, commonly found on articulated railcars. Building on lessons learned in developing road B-WIM, this paper describes the development and testing of a new railway bridge WIM system (RB-WIM). Unlike a previously reported study, trains were weighed statically to provide an evidence base for accuracy assessment. Changing speed was identified as a significant issue for trains and the algorithm was adapted to deal with it. To the best of the authors' knowledge, this is the first time that the constant velocity assumption has been replaced with an assumption that each individual carriage may have a different velocity. Constant velocity is a reasonable assumption for road vehicles but, given the length of a typical train, the velocity can change significantly during the train passage. The following sections provide details on how relative carriage velocity can impact the accuracy of the algorithm and how the algorithm was amended to account for the non-constant velocity. The system was developed as part of BridgeMon, a two-year research project funded under the European Commission's 7 th Framework programme. A steel truss bridge at Nieporet in Poland was used as a case study and to test the accuracy of the system.