A comparable question might be (and it would be interesting to know) did British Airways, when it was nationalised in say the 1970s, lease its aircraft or own them outright?
Of course that may be affected by conditions and methods in then the industry concerned stretching back to the time it was brought under state control - eg is state run airlines typically leased aircraft at the time the state started to run them, then that approach may well be continued with after nationalisation (were BOAC/Imperial Airways ever fully private sector businesses however?)
Did leasing of assets as a concept / financial model actually exist in the way we currently understand it when railways were nationalised in 1948? - when as is mentioned up thread UK railways were highly vertically integrated with the vast majority of their activity 'in house'.
Whenever there was gloom about the UK's balance of payments figures in the 1970s (every month basically) it was frequently pointed out that whenever BA took delivery of a purchased 747 it greatly affected the figures.
BR bought almost exclusively home products so didn't suffer from the same problem.
After exchange controls were lifted in 1979, and the £ was floated, those problems disappeared.
That, and the deregulation of banks and financial institutions in the 1980s, was when other forms of financing large purchases (like PFI) were allowed.
Most private airlines have a mix of owned and leased planes, but it's noticeable that when they fail financially you usually discover the assets are all leased.
Selling and leasing back is also common, to aid cash flow.
Thomas Cook tried that route but it didn't work.
There are now several business models for financing trains, TOCs are not obliged to use the 3 main Roscos.
Merseyrail's 777 fleet will be PTE-owned (Merseytravel), but they are still financed commercially.
Some fleet purchases cause a unique financial framework to be set up for them - I think Scotrail's 385s are in that category, also TfL's 345s.
Even the government's "cheap" borrowing has to be financed on the markets.
That all depends on the government's credit-worthiness - which is about to take a dive.