Our family did quite a few of these from Birmingham in the 70s and they often picked up at Northfield in the days before Cross-City. It must have been the only time that you could see 200 people standing there on the old centre platform. Did any of the other excursions pick up at minor stations?
One problem was that in the dark on the return the driver had to stop at an unfamiliar and sometimes unlit platform on a fast section of track. A couple of times we had to walk back through the train to get to a door that was at the platform, and once we overshot the platform entirely; I was young and tired and can't quite remember the details, but I think eventually the driver got permission to reverse back.
Don't remember what hauled the excursions either, but it was usually a long rake of Mark 1s. Our destination often seemed to be North Wales (Llandudno, Rhyl, Colwyn Bay), but I also remember York, a particularly grim November day in Scarborough, and Southend by a very convoluted route - there must have been others too.
Basically they were a cheap day out for families at the time.