Control of electronic interlocking blocks requires 100% reliable electronics as error-free operation of the railway switches is life-critical; to achieve a SIL 4 safety level, most of the safety measures are taken by software while the systems feature double or triple redundancy on the hardware side. A typical "2-out-of-3" system has three independent CPU cards with identical I/O functions using independent backplanes and power supplies - all of these components installed in the same 19" rack. At least two of the three computers must yield identical results to guarantee the correct operation of the complete system.

The computing equipment in this application is installed in hundreds of railway stations in Austria, Switzerland, Hungary, Spain, Iran and Scandinavia. It is based on standard CompactPCI® components in double Eurocard form factor. The computing core is a low-power Pentium® single-board computer which is replaced by a Celeron® M computer with PCI Express® technology after ten years. Since the official approval of this kind of equipment in Germany by the German railway authority is quite time-consuming, the minimum availability required for a certified system is ten years. Consequently, only Intel® components with a guaranteed five-year availability can be used.

Apart from a general rise in performance on the computing (1-GHz Celeron® M computer) and communication side (PCI Express® and Gigabit Ethernet), the number of I/Os required has not changed. The former six Fast Ethernet lines per CPU unit (in total 18 lines per signaling system) provided on PC-MIP® mezzanine modules are now realized as Gigabit Ethernet lines on the CPU itself and with XMC mezzanine modules. A single CPU board can nowadays already support up to 12 Gigabit Ethernets at full speed thanks to the PCI Express® technology. In both the old and in the new system M-Modules™ are used to provide the HDLC/SDLC functionality and PC-MIP® modules are used to provide the TTL I/O. A similar application for a different user features a single redundant Pentium® III based solution implemented in 3U CompactPCI® systems.