In message , David Hansen
writes

Does this mean that any brake application automatically cuts traction power?

That is my understanding and it has been stated by others who are in a
position to know. Such an interlock appears to have been common since
the 1960s.
Not on the 6300s, 800s,1000s,7000s,9500s,1600s or any steam engine I
ever worked on.
--
Clive.