LOOKING AFTER THE RAIL INFRASTRUCTURE
The yellow-liveried test trains of Network Rail, or ‘Infrastructure Monitoring’ (IM) trains to use the correct description, feature regularly in the Operations section of The Railway Magazine. Their varied motive power and unusual operating schedules certainly make them an attractive target for photographers. But the work they actually carry out is perhaps not so well understood. The equipment fitted to each of them varies so there is a degree of overlap between what certain train sets can achieve, but I will attempt to summarise the current situation.
Over the years, the remit of IM trains has extended from one of simply checking the alignment of the track to a range of processes to assess the wider railway.
Track geometry (or alignment) was originally checked and maintained by lengthmen, using gauge sticks, spirit levels and a skilled eye, but the ever-increasing speed and axle loading of trains in general required a more accurate and reliable method.
Early track recording coaches used mechanical displacement measuring equipment, and dropped paint onto the track to indicate any misalignments for correction by the lengthmen. However, their principal advantage was the fact they were carrying out measurements dynamically (i.e. with the weight of a train on the track). With their equipment properly calibrated, they also had the benefit of consistency – errors of judgement by any less experienced lengthman were eliminated.
Geometry issues
This is still true, though the extra safety benefit today, whether trains are running or not, is that it obviates the need to send people out onto the
