'Person ill on train'

It depends on what the illness was. If someone has some angina, or faints, then the train is unlikely to need to be cleaned. LU prefer the person to be be removed from the train at the station if realistically possible. This is because it usually makes treatment easier (you don't always want to be treating a heart attack in a cramped tube train), and is more likely to allow the service to continue when possible.

If the person has vomited in the car (or other messy things such as blood, other bodily fluids), then it is most likely that the train will be taken out of service. Although cars can be cut out, it is not done, as silly people may try and enter that carriage from other carriages. So the train would be completely removed from service.

When you have a person ill on a train, the first thing is obviously to assess how ill the person is. Hopefully they can stand and walk, thereby getting them off the train and allowing the service to continue with the minimum of delay. Then administering help as appropriate to the situation.

I dont think Inter-connecting doors can be "sealed up" either the car would be cut out or usually the whole train is taken out of service [as Stephenk has said]

Trains are also taken out of service if offensive graffiti has been sprayed [Sexist, Racist etc]