[jo909]

As far as I understand this is a historic safety mechanism.

Trains were very popular and important in the UK, transporting nearly the same amount of passenger pre World War 1 as they do today.

At this time no central locking existed and many train compartments, especially for the rich, had their own door that opens to the outside.

With handles on the inside it is much more likely to open by accident. The door contained the only window in the aisle between the seats of the compartment, where you would stand to look outside, so people naturally lean against it.

At the time central locking came around it wasn't necessarily reliable and people were already used to the situation, so there is little reason to suddenly introduce internal door handles again.