PRECAUTIONS WHILE RAILFANNING:
Posting a few out of my experience, plese post your suggestions to make a comprehensive list.
1. Always walk beside the track when taking photographs at single/double lines, best to make use of the Refugee area(Area with an R board) if available in case of tripod usage rather than using a tripod in uneven areas beside P.Way.
2. Its always good...
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to identify friendly RPF/GRP(can guess who are friendly by eavesdropping in their conversations :D), have a dailogue with them and gently ask them if the mind u taking a few pics, most of them wont after a few words. PR skills put to good use result in RPF thinking u r no harm and u getting photos with their consent, win-win situation. The trick of course lies in befriending them.
3. Alway lean your body against the door or put a bit of foot pressure on it while doorplating(Not supposed to do doorplating in the first place, but if you have to then follow this). You will block the door hitting you from behind at curves, even if you dont your body gets alerted due to the force of the door on your foot at curves.
4. Never be over confident on your skills while doorplating, centrifugal force can push one heck of a macho man out.Things go disturbingly wrong with overconfidence.
5. Be alert if watching trains enroute beside tracks, a distinct honk by an EMD/Electrics or chugging by an ALCO should give you an enough early warning to get a bit far from the track.
6. If questioned by RPF or railway staff try to ascertain them you are not a foe to railway, however if things backfire retreating and accepting mistake is the best way out.Never make the mistake of boasting of your "BADA BABU" Contacts if any, will not help you in all cases may actually make situations worse for you.
7. Its always good to have known company while doorplating or trying to take pics from a moving train, worst case somebody known would be there to help you in distress.
Please add, will keep adding myself too..

11. Never attempt to take pictures off platforms between tracks in stations, if nothing else you would draw more attention of security personnel.Try to draw least attention possible.
12. Never encourage people/railfans not so positive about doorplating. It is not really a proud capability to boast about.
13. Make a note of scenic sections in new routes and keep camera handy, dont hurry up at the last moment and loose control.
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Its not advisable to get down from trains if waiting for signal at home,especially if the train is on a curve. If you have to,get down towards the inner radius if the curve as boarding the train back from the outer radius would be tough coz the steps would be higher.
15.Avoid doorplating while its raining, door handles would be slippery.Paint over the handles makes the situation worse.
16. Never cross tracks for photography while train is halted in a loop in sections of heavy freight traffic, your train would probably be given a starter before the goods rake completely passes you and you may end up missing your train.
17. Not advisable doorplating while train is passing over a long bridge without metal bars, you never know when vertigo may strike you.
18. As AJ mentioned, keep tracking signals before crossing tracks while your train has halted for a crossing. Even if its an express train which is crossing, you cant really help it if it comes to a halt on the line you just crossed, again you may miss your train.