When re-signalling and permanent way works take place at the same time and over successive weekends, it is often necessary to install temporary site signals. Writes Peter Allan, principal project engineer, Invensys Rail

The drawback in using temporary signals in the past has always been the difficulties encountered when things go wrong – for example the whole railway has to be closed to replace a failed lamp. Without permanent safe-access arrangements, personnel access to these structures is restricted to mechanised plant (under Overhead Line Equipment isolation conditions), and so the performance impact of any failure, however unlikely, needs to be balanced against the potential engineering savings that accrue.

However, following the introduction of LED signals and their inherent reliability compared to traditional filament lamps, signal engineers are now able to propose lower cost, temporary solutions for short term installations.

Simple solutions

Some temporary signal installations can be quite simple structures. One typical example is a straight-post signal which was employed for several weeks on the Up line at Arkleston, near Paisley. Featuring a pre-cast concrete base, with lifting lugs and a basic straight post, the LED signal head is mounted along with the Signal Post Telephone and Signal Post Replacement Switch.

On the other hand, larger and more complicated examples, while taking longer to install (and remove), can result in significant cost savings. Engineers needed to provide a temporary signal on a four- way gantry on the Ayr line at Paisley. A temporary signal covering the left hand line (in the photograph) was attached to the gantry leg. It really was temporary – it had an operational life of only some 36 hours whilst the line remained in use. The alternative would have been to install a temporary gantry cage position, which would have added greatly to the cost of the structure.

Following the next permanent-way stage, the new line was brought into use and the final gantry cage installed. The temporary signal was removed and the completed, permanent installation commissioned.

These examples were both implemented by Invensys Rail on the Paisley Corridor Improvement project. There are many other examples where, in order to keep a line open during ongoing track realignments or signalling renewals, temporary signals are the economical answer. With further advances in LED technology and lightweight structures, there is likely to be wider use of these temporary signals in the future.