In message .com, MIG
writes
I think that a seven-car train would only have had three motor cars
though, wouldn't it, one of which would be in the middle somewhere?
I've never seen the picture of a control trailer (also in the middle
somewhere), but I assume that not too much space would be wasted.

And I think it would have been the underfloor equipment of 1938 stock
that allowed motors to be more distributed.
Northern line 38 stock had five motor cars per train.
--
Clive.