The number one danger of living on a floodplain is the real risk of your house flooding. With global warming, and rising sea levels, the risk of flood is increasing, which means that the danger of living on floodplains is higher now than it ever has been.
The difference between settlement and subsidence is if the movement of the building happened in spite of the weight of the building, then the associated movement of the building will be as a result of subsidence vs settlement. Whereas settlement is cause by the weight of the structure on the ground.
You should only buy a house next to a railway line once you have spent time listening to how noisy the trains are inside the house and in the garden. But you must consider future saleability of the house, as others will not like the sound of noisy trains on nearby train tracks.
You should only buy a house near a tube station if the location enhances the house’s desirability for commuters and the tubes cannot be heard from the house, unless noisy tube trains don’t trouble you. But you must consider future saleability, as others will not like the sound of noisy tube trains.
To avoid any regret buying a house on a busy street visit the house on a few occasions before you buy to assess the level of traffic and noise at different times of the day. Make sure to stand in the front and back garden, and ask if the vendor minds if you open the windows to assess traffic noise.
Only buy a house on a main road after considering road noise, the effects of pollution, the effect the proximity of the road will have on the future sale value and how difficult it will be to sell the house in the future. The closer to a main road the bigger the effect on value and saleability.