VCD is Vigilance control device. It is supposed to be present in all mainline locomotives & is used to ensure crew is not asleep during running of locomotive. At regular intervals, the cabin crew needs to press a button by foot, as a response to VCD. If not done, VCD will give warning & auto-brake. [ Deadman's handle is present in EMU/Multiple unit trains instead of VCD for same function, but the working is different ]

After a freight accident in CIC section where a GMO shed loco with a faulty...
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VCD & sleeping drivers dashed into another goods train some years back, Indian Railways made it mandatory for all sheds to check VCD before offering any loco for duty.

There are several types of Train Collision Avoidance System (TCAS), Kavach is one of them, developed indigenously by India. Kavach is a 'telecommunication' device & costs 50lakh/km. It has some functions like avoiding collision between two Kavach fitted trains on a Kavach enabled route, auto-whistle before LC gates, prevention of SPAD, prediction of upcoming signal aspect by voice (this feature already available in Vande Bharat) , prediction of upcoming TSR/PSR, reducing speed of train to 30kmph over turnouts, and many more.

Kavach is NOT concerned with keeping drivers awake, It is rather a pseudo-autopilot type system which will assist crew/take control of train when crew is not working properly.