In the 1960s, there were even relief trains run on Boxing Day, after a late start to services. Back then, most people had a lot less annual leave (15 days was typical- some got only 12 or 13 days). Those like me caught the last train north out of London Euston on Christmas Eve, and returned south on the late afternoon departure from Liverpool on Boxing Day, because we had to work on 27 December (in years when it was not a Bank Holiday) - and trains were well loaded.
It was not until the 1970s / 1980s that lots of people got enough annual leave to stay off work between Christmas & New Year - eventually a lot of businesses (other than shops, etc.) decided that it was not worth opening when so few staff wanted to work over this period.