>  anti collision system is to be used in loco moving on same rail between
> points in a small area... 

Ah, OK, that gives some idea of scale. If you went with ultrasonics,
probably not a bad choice, fairly simple to implement

I think I'd tend to over-engineer (more-than-adequately-engineer ?)
this and use a piezo tweeter for the transmitter, which would need
40kHz power drive. Quite doable, the basis of ultrasonic cleaners.
Something bigger than the Tx/Rx pair you can buy for robotics
anyway. You might even code the 40kHz to ensure good detection
and test both reflection time and signal identity

It's possible that a car wrecker might have an ultrasonic reversing
unit found in a lot of late model vehicles, could be a cheap way to
experiment

-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist