if you look at the accounts/statistics: staff is the majority of the cost

the official figures here are however misleaading as their "staff" numbers only include those directly employed by TOCs and Network Rail

under "other" there's the vast army of contractors, including cleaning staff, track maintenance contractors, train maintenance contractors, private security, and so on

what's left is fuel, interest payments, depreciation, premium payments to government and some capital expenditure (not much...)