<P> IR primarily uses coloured signal lights, which replaced semaphores and disc - based signalling (dependent on position or colour). IR uses two - aspect, three - aspect and four (or multiple) aspect color signalling across its network . </P> <P> Signals at most stations are interlocked using panel interlocking, route - relay interlocking or electronic interlocking methods that eliminate scope for human signalling errors . IR uses track circuiting, and block proving axle counters for train detection . </P> <P> As of March 2017, 5584 stations across IR (about 90% of stations on broad gauge) have interlocked stations and multi-aspect signalling . Around 99% of key routes (A, B, C and D) have track circuitry or block proving axle counters for automated train detection . Also, IR has about 51,000 route kilometers of optical fiber cable network across India, that is used for train control, voice and data communication . Around 2,500 km (1,600 mi) of the route is covered by GSM - R based Mobile Train Radio Communication . </P> <P> In December 2017, IR announced that it will implement ETCS Level 2 system for signalling and control on key routes with an investment of ₹ 12,000 crore (US $1.7 billion). Currently IR uses Centralised Traffic Control (CTC) on the busy Ghaziabad - Kanpur route and real - time train monitoring systems on Mumbai and Kolkata suburban routes . </P>

Which state of india does not have a railway line