Anti Collision Device

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The Anti Collision Device is a form of automatic train protection invented by and used on Indian Railways.


The ACD Network is a Train Collision prevention system invented by Rajaram Bojji and patented by Konkan Railway Corporation Limited (a public-sector undertaking of the Ministry of Railways, Government of India). ACDs have knowledge embedded intelligence. They take inputs from GPS satellite system for position updates and network among themselves for exchanging information using their data radio modems to take decisions for timely auto-application of brakes to prevent dangerous 'collisions', thus forming a 'Raksha Kavach' (meaning a '(Train) Safety shield').

ACDs fitted (both in locomotive and guard's van of a train) act as a watchdog in the dark as they constantly remain in lookout for other train bound ACDs, within the braking distance required for their relative speeds. They communicate through their radios and identify each other. If they happen to find themselves on the same track and coming closer to each other, they automatically restrain and stop each other, thereby preventing dangerous head-on and rear-end collisions.

Loco ACD of a train also applies brakes to reduce the train speed either to 15 km/h if on approach it receives a message from other train bound ACD that has stopped in a block section on adjacent track (and driver of that train has yet not communicated that things are 'Normal') or to bring the train to a stop if train bound ACDs of other train are radiating 'train parted' message thereby preventing dangerous side collision that may occur due to infringement of adjacent track by a stopped or a 'parted' train, respectively.

ACD trials have recently been concluded successfully in Southern Railway. Further implementation on Indian Railway is awaited.

Level crossings[edit]

Loco ACD on receipt of 'Gate Open' input from Gate ACD (provided at non-interlocked level crossing), applies brake to regulate its train speed to 30 km/h or as per requirement previously set. Gate ACDs fitted at manned and unmanned level crossings also give audio-visual 'Train Approach' warning to road users if an ACD fitted train approaches them.

Also if a Loco ACD receives manual 'SOS' message from other Train bound ACD or a Station ACD that is within three kilometres of its radial range; it applies brakes automatically to bring the train to a stop.

The application of this Anti-Collision Device has been refined to not only prevent 'midsection' collisions but also to prevent their occurrences in 'station yards'. The newly engineered solution is integrated with the signalling systems and interlocking to react appropriately in case collision-like conditions are perceived at the time of reception and dispatch of trains from a station (e.g. while approaching a station), Loco ACD also gives 'Station Approach' alert to the driver and regulates its train speed when it receives information from Station ACD to this effect, namely, either mainline is occupied by a train or a load stabled on it and not fitted with ACD or if route for train reception is not set for mainline.

Loco shed ACDs, Track-ID Assigning ACDs and Repeater ACDs further to strengthen the working of ACD Network.


Pilot project of "Provision of ACD Network" has been successfully commissioned recently on the Northeast Frontier Railway (of Indian Railways), covering 1,736 kilometres (1,079 mi) of its Broad Gauge route. Final commissioning trials of ACD network installed on 760 kilometres (470 mi) of Konkan Railway route is presently underway.

Anti Collision Device (ACD), which is an on-board train protection device and also the first ever device in the world indigenously developed by Konkan Railway with their Technical Partner Kernex Microsystems (I) Ltd, will be in place by 2013 on the entire Indian Railway network so as to reduce chances of Train collisions.

A new ACD Version-II [now called Train Collision Avoidance System (TCAS)] is under development by The Research Designs and Standards Organisation (RDSO). Unlike ACD which is more of a distributed system which acts independently, the TCAS will be more of a centralized system where in Station TCAS controls communication between locos and with locos with TDMA protocol. The TCAS under development is meant to be a vital safety system meaning TCAS have a deep coupling with Railway's signalling system where as ACD systems does not depend on Railway's signalling system.

ACD Deficiencies[edit]

The ACD system is based on GPS based positioning and track detection. This had its inherent problems as with the GPS- Standard Positioning GPS service or Coarse acquisition (Precision positioning is only available in US for military use) the best possible horizontal accuracy is 10 m. This is inadequate for detection of rail tracks separated by a distance of 10–15 feet. ACD does not even have DGPS (Differential GPS that gives an accuracy close to 2.5m ) and hence had errors in track detection using their patented Deviation Count Theory that worked in block sections but failed in station sections. The result was erratic braking that disrupted train movements and proved to be ineffective.[1]

Another design different to ACD was patented in 2001, named "Railway Collision Avoidance System" by an Indian inventor, Indranil Majumdar from Calcutta who was awarded the Texas Instruments Analog Design Challenge 2001 for this design and the patent granted in 2007.[2] The design ruled out GPS as it was a 3rd party system (US based) and suggested track based sensors similar to RFID (Radio Frequency Identification Device) or Balise (similar to the EuroBalise). The design didn't receive much attention as the inventor had no railway background. However, the design concepts were eventually reflected in the TCAS design first released in 2008.

After seven or eight years of problems with the ACD system, RDSO, Lucknow drafted the Train Collision Avoidance System (TCAS) specs. with amendments that followed and finally in 2012, the Ver3.1.1 spec. has been released (after joint consultation with companies manufacturing signaling equipment for the Indian Railways). The ACD system though in use with the Indian Railways, has its inherent problems in Station Sections due to its design concept of using GPS for track detection that is not viable.

The High Level Safety Review Committee at Mumbai on 12/13 January 2012 at the Western Railway HQ were sceptic of the ACD effectivity and were unanimous of having TCAS developed, as an open architecture system that shall offer multi-vendor operability without attracting any royalty unlike the ACD which is proprietary.[3]

TCAS being developed by qualified companies (RDSO approved and manufacturing railway safety systems that includes, Kernex Microsystems, Medha Servo Drives Hyderabad, Invensys Bangalore, Siemens, HBL Power Systems Ltd Hyderabad and others) in India selected by RDSO through an Expression of Interest (EOI), shall be an Indian Train Protection System offering collision avoidance and also offer many functionalities of the European Train Control System that includes prevention of Signal Passing at Danger (SPAD), Movement Authority and Control, Critical Train Data Recorder, advance in cab display of signals, advance alerts and warnings from Station sections, uploading of running train data to a Central Train Management System over GSM-GPRS or other cellular networks.

TCAS has been selected for implementation and it is going to cost very heavy to Govt as per this news report there would be a cost of 10 L INR per kilometre.[4]


  1. ^ As per the CAG report the ACD is not foolproof and has inherent deficiencies.
  2. ^ India 201106 
  3. ^ Kakodkar, Anil; Sreedharan, E.; Vedachalam, N.; Dhande, Sanjay G.; Srivastava, G.P.; Amitabh (February 2012). Report of High Level Safety Review Committee (Report). New Delhi: Government of India Ministry of Railways. Retrieved 2014-10-11. 
  4. ^ [1]