In railways, computer-based interlocking (CBI) systems are safety critical systems, and should be proved that there is no defect leading to catastrophes during their whole life cycles. Runtime verification is able to provide on-going protection during the operational phase. Unfortunately, as being a variantrich system, it is hard to build monitors for CBI with standard runtime verification frameworks. To overcome this shortcoming, we propose parametric timed automata (PTA) to build feature models for the system. We prove that the language of PTA has more expressive power than timed automata. Based on PTA, we build monitors for a concrete CBI used in Beijing Metro Line 7. The experiment results show that complicated variant-rich CBI systems can be specified with a unique PTA feature model, and monitors can be quickly derived from the model by assign values to parameters. Therefore, this approach is feasible for monitoring CBI systems.
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