On commuter trains and trams, the big problem is free newspapers, in particular after rush hour. Certainly with trams, the newspaper suppliers have an arrangement where they remove them from the trams. Otherwise, litter is generally modest on urban transport, but I think they could do with having some roving cleaning staff especially after rush hour.
On intercity trains, there are on-board cleaning staff sometimes. The big problem tends to be food and drink packaging left on tables. The problem can sometimes be compounded by trains not being cleaned at termini.
Stations and stops tend to only be modestly littered, although stop stations have a problem with cigarette litter on the tracks.
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