I usually forget one item every time I get on the train for a photographic day trip, so I just made a useful definitive list. It can all fit in a Lowepro Slingshot 100AW bag, with the exception of newspapers, which can be carried in a carrier bag and discarded en route.
Here's the list:
– Bottle of tapwater – 500 ml
– Some food, including banana
– Plasters – in case of blisters
– Diarrhoea tablets (any brand) – you can't be too careful, can you?
– Blower brush – currently lost
– Lens cloth
– Digital SLR (Nikon D90) plus kit zoom
– 28 mm lens
– Spare memory card
– A big pile of really old newspapers that I never got round to looking at. For reading on the train, and useful for creating that mad-person-on-the-train vibe.
– Map (if I have one of the place I'm visiting). Very important – an index of possibilities.
– Mystery item
– MP3 player. Sometimes. I like music but don't like listening to it on headphones, so it’s mainly for emergency use on long journeys; for example if the train carriage is invaded by pissed-up Pompey supporters or someone with an even louder MP3 player.
Footnote. If travelling on First Great Western trains from Weymouth to Bristol, extra equipment is necessary, such as a torch for when the lights fail, extra food in case of delays and, in winter, emergency clothing for when there's no heating. Tools can come in useful when bits of the train fall off, as in my last journey where the door between the two carriages could not be opened as the guard had kept the door handle for safe keeping. Also the one toilet may be out of order for the 2.5 hour trip, so a large bottle can come in handy in emergencies...
I think that's it.