On top of everything BestWestern said, in no particular order I carry my raincoat (issue), spare torch, spare batteries for all my torches, needle & thread, spare buttons for jacket & shirt, flask/cup (in cold weather I keep it filled), knife/fork/spoon, plastic bowl, tea&coffee making, cuppa soup, paracetamol, sticking plasters, cable ties, gloves (leather & protective), pens, magic marker, USB charger with cable for blackberry, headphones (my phone turns into radio, it came in useful during 7/7 bombings and a fortnight later to keep up with news on-train), route maps, two bottles of water (one for me, one for somebody else), insulating tape, screwdriver & bits, scissors, pliers, mints, mars bar (for emergency use!), book to read when sitting, spit kit (for gathering evidence if anyone spits at/on us), spare T-key, train keys & whistle. Oh, and my lunch will be in there as well.
I think that's all. My bag weighs approx 14kg, by the way (most of which is my lunch!)
That's ridiculous! If there's one thing the Internet has taught me about guards, it's that they only need a newspaper and a place to rest their feet.