Not just 70s and 80s... I went on a Pendolino with a couple of one-time pendo fleet engineers: we were sat in coach H (for horrible.) At every neutral section there was a bang as the power was cut out and then two more distinct bangs as two circuit breakers operated. It gave them great pleasure to hear, it really annoyed me, to the point that I usually try to avoid the S end of that coach! I don't understand why the two breakers operate as the power is re-connected - that's if I understood what was happening correctly.
A 9-car Pendolino has 3 VCBs, one for connecting each pantograph (on coaches C and H) and the third (also on coach H)to energise the transformer at the other end of the train via the 25kV roof cable (busline). The train runs with a single pantograph raised, usually the rear one.
In normal formation (1st class at London end) the pantograph on coach C will be raised heading south and the pantograph on coach H will be raised heading north. Heading south the main VCB is closed on coach C and the busline VCB is closed on coach H. Heading north the main VCB and busline VCB on coach H are both closed.
When the train approaches a neutral section the APC (Automatic Power Control) opens all VCBs simultaneously at the first APC magnet. After crossing the neutral section the train passes the second APC magnet and the VCBs re-close sequentially (main VCB followed 4 seconds later by the busline VCB). This is to minimise the inrush currents when the transformers are energised.
If you are sat in coach H then when running south you will hear the busline VCB open and re-close at each neutral section. However when running north you will hear the main VCB and busline VCB open together as entering the neutral section and then the main VCB close followed by the busline VCB on exiting the neutral section.
If you are sat in coach C you will hear the main VCB operate at neutral sections when running south but no VCB operation when running north.
(An 11-car Pendolino has a further VCB on coach U that closes 4 seconds after the busline VCB)