"Roland Perry" wrote

It would probably be more secret.

No-one knows where most of the Royals are anyway. Smuggling them out by help
or car seems less conspicuous than hijacking a train and seizing exclusive
use of part of the network for the necessary time.

And it depends what the risk is. If it
was a dirty biological bomb in London, and a riot amongst the populace who
were being prevented from leaving for fear of infecting the rest of the
country, it could well be the best of the three options.

But if mass panic has hit London, the Tube network will not be running
anyway.