A bicycle! Not sure what its like now but 8/9 years ago, carrying a bicycle on the London underground was pretty normal. After arriving in Japan, my wife bought a bicycle in Okachimachi [御徒町] near Akihabara. She then tried to carry it onto the train and was told that it was forbidden - carrying a foldable compact bike onto the trains however is OK.
Luckily, me wife is rather good with her bearings and managed to cycle from east Tokyo all the way to Higashi Fushimi in West Tokyo 28 kilometers away. I was surprised not at the distance, but at the fact that she made it through this concrete spaghetti jungle without a map or one of these.
Are you allowed to carry bicycles on to the train in your region?
Below: Taken at Narita airport after coming back from LA.
More rules. No smoking, no running on to the train, no lying on the floor or getting drunk, no selling stuff, no bombs, no singing and no begging.